Directional display apparatus

ABSTRACT

A switchable privacy display comprises a spatial light modulator (SLM), a first switchable liquid crystal (LC) retarder and first passive retarder between a first pair of polarisers and a second switchable LC retarder and second passive retarder between a second pair of polarisers. The first switchable LC retarder comprises a homeotropic alignment layer and a homogeneous alignment layer. The second switchable LC crystal retarder comprises two homeotropic alignment layers or two homogeneous alignment layers. In landscape or portrait privacy mode, on-axis light from the SLM is directed without loss, whereas off-axis light has reduced luminance to reduce visibility to off-axis snoopers. Display reflectivity may be reduced for on-axis reflections of ambient light, while reflectivity may be increased for off-axis light to achieve increased visual security. In public mode, the LC retardance is adjusted so that off-axis luminance and reflectivity are unmodified. The display may switch between day-time and night-time operation.

TECHNICAL FIELD

This disclosure generally relates to illumination from light modulationdevices, and more specifically relates to optical stacks for providingcontrol of illumination for use in display including privacy display andnight-time display.

BACKGROUND

Privacy displays provide image visibility to a primary user that istypically in an on-axis position and reduced visibility of image contentto a snooper, that is typically in an off-axis position. A privacyfunction may be provided by micro-louvre optical films that transmitsome light from a display in an on-axis direction with low luminance inoff-axis positions. However such films have high losses for head-onillumination and the micro-louvres may cause Moiré artefacts due tobeating with the pixels of the spatial light modulator. The pitch of themicro-louvre may need selection for panel resolution, increasinginventory and cost.

Switchable privacy displays may be provided by control of the off-axisoptical output.

Control may be provided by means of luminance reduction, for example bymeans of switchable backlights for a liquid crystal display (LCD)spatial light modulator. Display backlights in general employ waveguidesand edge emitting sources. Certain imaging directional backlights havethe additional capability of directing the illumination through adisplay panel into viewing windows. An imaging system may be formedbetween multiple sources and the respective window images. One exampleof an imaging directional backlight is an optical valve that may employa folded optical system and hence may also be an example of a foldedimaging directional backlight. Light may propagate substantially withoutloss in one direction through the optical valve whilecounter-propagating light may be extracted by reflection off tiltedfacets as described in U.S. Pat. No. 9,519,153, which is hereinincorporated by reference in its entirety.

BRIEF SUMMARY

According to a first aspect of the present disclosure there is provideda display device comprising: a spatial light modulator; a displaypolariser arranged on a side of the spatial light modulator, the displaypolariser being a linear polariser; a first additional polariserarranged on the same side of the spatial light modulator as the displaypolariser, the first additional polariser being a linear polariser; atleast one first polar control retarder arranged between the firstadditional polariser and the display polariser, a second additionalpolariser, the second additional polariser being a linear polariser; andat least one second polar control retarder, wherein either: the secondadditional polariser is arranged on the same side of the spatial lightmodulator as the first additional polariser outside the first additionalpolariser, and the at least one second polar control retarder isarranged between the first additional polariser and the secondadditional polariser; or the display device further comprises abacklight arranged to output light, the spatial light modulatorcomprises a transmissive spatial light modulator arranged to receiveoutput light from the backlight, said display polariser is an inputdisplay polariser arranged on the input side of the spatial lightmodulator, and the display device further comprises an output displaypolariser arranged on the output side of the spatial light modulator,the second additional polariser is arranged on the output side of thespatial light modulator, and the at least one second polar controlretarder is arranged between the second additional polariser and theoutput display polariser, wherein: each of the at least one first polarcontrol retarder and the at least one second polar control retardercomprises a respective switchable liquid crystal retarder comprising alayer of liquid crystal material and two surface alignment layersdisposed adjacent to the layer of liquid crystal material and onopposite sides thereof, in respect of one of the at least one firstpolar control retarder and the at least one second polar controlretarder, one of the surface alignment layers is arranged to providehomogenous alignment in the adjacent liquid crystal material and theother of the surface alignment layers is arranged to provide homeotropicalignment in the adjacent liquid crystal material, and in respect of theother of the at least one first polar control retarder and the at leastone second polar control retarder, both of the surface alignment layersare arranged to provide homogenous alignment in the adjacent liquidcrystal material or both of the surface alignment layers are arranged toprovide homeotropic alignment in the adjacent liquid crystal material.Advantageously a switchable privacy display may be provided withextended polar regions over which desirable security level may beachieved.

The switchable liquid crystal retarder of said one of the at least onefirst polar control retarder and the at least one second polar controlretarder may have a retardance for light of a wavelength of 550 nmhaving a first retardance value in a range from 700 nm to 2500 nm,preferably in a range from 850 nm to 1800 nm. Advantageously luminancemay be reduced over desirable polar regions; in embodiments comprising areflective polariser reflectance may be increased over a wide polar areaand security level may be provided at desirable levels over a wide polarregion.

Said one of the at least one first polar control retarder and the atleast one second polar control retarder may further comprise at leastone passive compensation retarder. Advantageously the area of luminancereduction may be increased. Further the uniformity of luminance inpublic mode may be increased.

The spatial light modulator may comprise an emissive spatial lightmodulator arranged to output light, the display polariser may be anoutput display polariser arranged on the output side of the emissivespatial light modulator, the second additional polariser may be arrangedon the output side of the spatial light modulator outside the firstadditional polariser, and the at least one second polar control retardermay be arranged between the first additional polariser and the secondadditional polariser. Advantageously a switchable privacy display withhigh security factor may be provided for an emissive spatial lightmodulator.

The display device may further comprise a backlight arranged to outputlight, and the spatial light modulator may comprise a transmissivespatial light modulator arranged to receive output light from thebacklight. Advantageously a switchable privacy display with highsecurity factor may be provided for a transmissive spatial lightmodulator.

The at least one passive compensation retarder of said one of the atleast one first polar control retarder and the at least one second polarcontrol retarder may be arranged on the same side of the switchableliquid crystal retarder as the surface alignment layers that is arrangedto provide homeotropic alignment in the adjacent liquid crystalmaterial. The at least one passive compensation retarder of said one ofthe at least one first polar control retarder and the at least onesecond polar control retarder may comprise a passive uniaxial retarderhaving its optical axis perpendicular to the plane of the retarder. Thepassive uniaxial retarder may have a retardance for light of awavelength of 550 nm in a range from −400 nm to −2100 nm, preferably ina range from −700 nm to −1700 nm. Advantageously the size of the polarregion for desirable security level is increased.

The display device may further comprise a reflective polariser, thereflective polariser being a linear polariser, and either: said displaypolariser may be an output display polariser arranged on the output sideof the spatial light modulator, the second additional polariser may bearranged on the same side of the spatial light modulator as the firstadditional polariser outside the first additional polariser, the atleast one second polar control retarder may be arranged between thefirst additional polariser and the second additional polariser, and thereflective polariser may be arranged between the first additionalpolariser and the at least one second polar control retarder; or thedisplay device may further comprise a backlight arranged to outputlight, the spatial light modulator may comprise a transmissive spatiallight modulator arranged to receive output light from the backlight,said display polariser may be an input display polariser arranged on theinput side of the spatial light modulator, and the display device mayfurther comprise an output display polariser arranged on the output sideof the spatial light modulator, the second additional polariser may bearranged on the output side of the spatial light modulator, the at leastone second polar control retarder may be arranged between the secondadditional polariser and the output display polariser, and thereflective polariser may be arranged between the output displaypolariser and at least one second polar control retarder. Advantageouslyin a privacy mode of operation, increased display reflectivity may beprovided. In ambient illuminance, increased security level of thedisplay may be achieved for snooper locations.

Said one of the at least one first polar control retarder and the atleast one second polar control retarder may be the at least one secondpolar control retarder and said other of the at least one first polarcontrol retarder and the at least one second polar control retarder maybe the at least one first polar control retarder.

The switchable liquid crystal retarder of said one of the at least onefirst polar control retarder and the at least one second polar controlretarder may have a retardance for light of a wavelength of 550 nmhaving a first retardance value and the switchable liquid crystalretarder of said other the at least one first polar control retarder andthe at least one second polar control retarder may have a retardance forlight of a wavelength of 550 nm has a second retardance value, the firstretardance value being greater than the second retardance value. Themagnitude of the difference between half the first retardance value andthe second retardance value may be at most 400 nm. The first and secondpolar control retarders may provide luminance reduction and reflectionincrease for different polar regions. Advantageously the polar regionover which desirable visual security is achieved is increased.

In respect of said other of the at least one first polar controlretarder and the at least one second polar control retarder, both of thesurface alignment layers may be arranged to provide homogenous alignmentin the adjacent liquid crystal material which has a retardance for lightof a wavelength of 550 nm in a range from 450 nm to 900 nm, preferablyin a range from 550 nm to 800 nm, and said other of the at least onefirst polar control retarder and the at least one second polar controlretarder may further comprise a pair of passive uniaxial retardershaving optical axes in the plane of the retarders that are crossed andeach having a retardance for light of a wavelength of 550 nm in a rangefrom 250 nm to 800 nm, preferably in a range from 400 nm to 625 nm. Inrespect of the other of the at least one first polar control retarderand the at least one second polar control retarder, both of the surfacealignment layers may be arranged to provide homogenous alignment in theadjacent liquid crystal material which has a retardance for light of awavelength of 550 nm in a range from 500 nm to 900 nm, preferably 600 nmto 850 nm, and said other of the at least one first polar controlretarder and the at least one second polar control retarder may furthercomprise a passive uniaxial retarder having its optical axisperpendicular to the plane of the retarder and having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −700 nm,preferably in a range from −350 nm to −600 nm. In respect of the otherof the at least one first polar control retarder and the at least onesecond polar control retarder, both of the surface alignment layers maybe arranged to provide homeotropic alignment in the adjacent liquidcrystal material which has a retardance for light of a wavelength of 550nm in a range from 500 nm to 900 nm, preferably in a range from 600 nmto 850 nm, and said other of the at least one first polar controlretarder and the at least one second polar control retarder may furthercomprise a passive uniaxial retarder having its optical axisperpendicular to the plane of the retarder and having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −900 nm,preferably in a range from −400 nm to −800 nm. Advantageously highluminance reduction and reflection may be achieved for polar regionsthat are complementary to the luminance reduction of one of the at leastone first polar control retarder and the at least one second polarcontrol retarder. The security level may be increased and the size ofthe polar region for desirable visual security may be increased.

The spatial light modulator may comprise an emissive spatial lightmodulator arranged to output light, the display polariser may be anoutput display polariser arranged on the output side of the emissivespatial light modulator, the second additional polariser may be arrangedon the output side of the spatial light modulator outside the firstadditional polariser, and the at least one second polar control retarderis arranged between the first additional polariser and the secondadditional polariser. Advantageously an emissive display may be providedwith desirable security level over a desirable polar range.

The emissive spatial light modulator may have an output luminanceprofile having a full width half maximum that is at least 40 degrees,preferably at least 50 degrees. Advantageously the public mode ofoperation may be provided with high image visibility over a wide polarrange.

The display device may further comprise a backlight arranged to outputlight, and the spatial light modulator may comprise a transmissivespatial light modulator arranged to receive output light from thebacklight. The backlight may have an output luminance profile having afull width half maximum that is at least 40 degrees, preferably at least50 degrees.

The switchable liquid crystal retarder of said one of the at least onefirst polar control retarder and the at least one second polar controlretarder may have a retardance for light of a wavelength of 550 nmhaving a first retardance value and the switchable liquid crystalretarder of said other the at least one first polar control retarder andthe at least one second polar control retarder may have a retardance forlight of a wavelength of 550 nm has a second retardance value, half ofthe first retardance value being less than the second retardance value.In respect of the other of the at least one first polar control retarderand the at least one second polar control retarder, both of the surfacealignment layers may be arranged to provide homogenous alignment in theadjacent liquid crystal material which has a retardance for light of awavelength of 550 nm in a range from 700 nm to 2500 nm, preferably in arange from 850 nm to 1800 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder may further comprise a pair of passive uniaxial retardershaving optical axes in the plane of the retarders that are crossed andeach having a retardance for light of a wavelength of 550 nm in a rangefrom 600 nm to 1600 nm, preferably in a range from 750 nm to 1300 nm. Inrespect of the other of the at least one first polar control retarderand the at least one second polar control retarder, both of the surfacealignment layers may be arranged to provide homeotropic alignment in theadjacent liquid crystal material which has a retardance for light of awavelength of 550 nm in a range from 700 nm to 2500 nm, preferably in arange from 1000 nm to 1800 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder may further comprise a passive uniaxial retarder having itsoptical axis perpendicular to the plane of the retarder and having aretardance for light of a wavelength of 550 nm in a range from −700 nmto −2500 nm, preferably in a range from −900 nm to −1800 nm.Advantageously a switchable privacy display may be provided to achieve anarrow privacy switch-on angle.

The spatial light modulator may comprise an emissive spatial lightmodulator arranged to output light, the display polariser may be anoutput display polariser arranged on the output side of the emissivespatial light modulator, the second additional polariser may be arrangedon the output side of the spatial light modulator outside the firstadditional polariser, and the at least one second polar control retardermay be arranged between the first additional polariser and the secondadditional polariser. The emissive spatial light modulator may comprisean array of pixels arranged in a pixel layer, and the display device mayfurther comprise a parallax barrier forming an array of apertures,wherein the parallax barrier is separated from the pixel layer by aparallax distance along an axis along a normal to the plane of the pixellayer, each pixel being aligned with an aperture. The emissive spatiallight modulator and the parallax barrier may have an output luminanceprofile having a full width half maximum that is at most 40 degrees.Advantageously a switchable privacy display may be provided withincreased security level for off-axis snooper locations. An emissivedisplay may be provided to operate in a privacy mode for both landscapeand portrait operation.

The display device may comprise a backlight arranged to output light,and the spatial light modulator may comprise a transmissive spatiallight modulator arranged to receive output light from the backlight. Thebacklight may have an output luminance profile having a full width halfmaximum that is at most 40 degrees. Advantageously a privacy display maybe provided for a transmissive displays. The backlight may be providedwith reduced cone angle. The polar area for desirable security level maybe increased.

Said display polariser may be an input display polariser arranged on theinput side of the spatial light modulator; the first additionalpolariser may be arranged between the backlight and the input displaypolariser; and the second additional polariser may be arranged on thesame side of the spatial light modulator as the first additionalpolariser between the backlight and the first additional polariser, andthe at least one second polar control retarder may be arranged betweenthe first additional polariser and the second additional polariser.Advantageously the visibility of frontal reflections from the frontsurface of the display device may be reduced.

Said surface alignment layers of said one of the at least one firstpolar control retarder and the at least one second polar controlretarder may have pretilts having pretilt directions with components inthe plane of the layer of liquid crystal material in a first pair ofanti-parallel directions, and said surface alignment layers of saidother of the at least one first polar control retarder and the at leastone second polar control retarder may have pretilts having pretiltdirections with components in the plane of the layer of liquid crystalmaterial in a second pair of anti-parallel directions, the first pair ofanti-parallel directions being crossed with the second pair ofanti-parallel directions. The first pair of anti-parallel directions maybe at 90 degrees to the second pair of anti-parallel directions, asviewed normal the planes of the layers of liquid crystal material of theat least one first polar control retarder and the at least one secondpolar control retarder. Advantageously polar regions with desirablesecurity level may be achieved in both lateral and elevation directions.A switchable privacy display may be provided in landscape and portraitdirections. In an automotive use, reflections from windscreens may bereduced.

The switchable liquid crystal retarder of said one of the at least onefirst polar control retarder and the at least one second polar controlretarder may have a retardance for light of a wavelength of 550 nmhaving a first retardance value and the switchable liquid crystalretarder of said other the at least one first polar control retarder andthe at least one second polar control retarder may have a retardance forlight of a wavelength of 550 nm has a second retardance value, the firstretardance value being greater than the second retardance value. Themagnitude of the difference between half the first retardance value andthe second retardance value is at most 400 nm. Advantageously the sizeof the polar region for desirable security level is increased.

In respect of the other of the at least one first polar control retarderand the at least one second polar control retarder, both of the surfacealignment layers may be arranged to provide homogenous alignment in theadjacent liquid crystal material which has a retardance for light of awavelength of 550 nm in a range from 450 nm to 900 nm, preferably in arange from 550 nm to 800 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder further comprises a pair of passive uniaxial retarders havingoptical axes in the plane of the retarders that are crossed and eachhaving a retardance for light of a wavelength of 550 nm in a range from250 nm to 800 nm, preferably in a range from 400 nm to 625 nm. Inrespect of the other of the at least one first polar control retarderand the at least one second polar control retarder, both of the surfacealignment layers may be arranged to provide homogenous alignment in theadjacent liquid crystal material which has a retardance for light of awavelength of 550 nm in a range from 500 nm to 900 nm, preferably in arange from 600 nm to 850 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder may further comprise a passive uniaxial retarder having itsoptical axis perpendicular to the plane of the retarder and having aretardance for light of a wavelength of 550 nm in a range from −300 nmto −700 nm, preferably in a range from −350 nm to −600 nm. In respect ofthe other of the at least one first polar control retarder and the atleast one second polar control retarder, both of the surface alignmentlayers may be arranged to provide homeotropic alignment in the adjacentliquid crystal material which has a retardance for light of a wavelengthof 550 nm in a range from 500 nm to 900 nm, preferably in a range from600 nm to 850 nm, and said other of the at least one first polar controlretarder and the at least one second polar control retarder may furthercomprise a passive uniaxial retarder having its optical axisperpendicular to the plane of the retarder and having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −800 nm,preferably in a range from −400 nm to −800 nm.

The spatial light modulator may comprises an emissive spatial lightmodulator arranged to output light, the display polariser is an outputdisplay polariser arranged on the output side of the emissive spatiallight modulator, the second additional polariser is arranged on theoutput side of the spatial light modulator outside the first additionalpolariser, and the at least one second polar control retarder isarranged between the first additional polariser and the secondadditional polariser. Said other of the at least one first polar controlretarder and the at least one second polar control retarder further maycomprise at least one passive compensation retarder.

The at least one passive compensation retarder of said other of the atleast one first polar control retarder and the at least one second polarcontrol retarder may comprise a passive uniaxial retarder having itsoptical axis perpendicular to the plane of the retarder. Advantageouslythickness and cost may be reduced. Advantageously the polar region overwhich desirable security level is achieved is increased in the case thatthe polar control retarder further comprises two homeotropic alignmentlayers.

The at least one passive compensation retarder of said other of the atleast one first polar control retarder and the at least one second polarcontrol retarder may comprise a pair of passive uniaxial retardershaving optical axes in the plane of the retarders that are crossed.Advantageously the polar region over which desirable security level isachieved is increased in the case that the polar control retarderfurther comprises two homogeneous alignment layers.

In respect of said other of the at least one first polar controlretarder and the at least one second polar control retarder, both of thesurface alignment layers may be arranged to provide homeotropicalignment in the adjacent liquid crystal material. Advantageously powerconsumption in public mode of operation is reduced.

The display device may further comprise a backlight arranged to outputlight, the spatial light modulator comprises a transmissive spatiallight modulator arranged to receive output light from the backlight, andsaid other of the at least one first polar control retarder and the atleast one second polar control retarder may be between the backlight andthe transmissive spatial light modulator. The thickness of componentsarranged on the front of the display is reduced. Increased front surfacediffusion may be provided with low pixel blurring, increasing imagefidelity and reducing the visibility of specular reflections.

In respect of said other of the at least one first polar controlretarder and the at least one second polar control retarder, both of thesurface alignment layers may be arranged to provide homogeneousalignment in the adjacent liquid crystal material. Advantageously thevisibility of artefacts under applied pressure may be reduced. The polararea for desirable security level may be increased.

Any of the aspects of the present disclosure may be applied in anycombination.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiments may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audio-visual systems and electrical and/oroptical devices. Aspects of the present disclosure may be used withpractically any apparatus related to optical and electrical devices,optical systems, presentation systems or any apparatus that may containany type of optical system. Accordingly, embodiments of the presentdisclosure may be employed in optical systems, devices used in visualand/or optical presentations, visual peripherals and so on and in anumber of computing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1A is a schematic diagram illustrating in side perspective view aswitchable privacy display for use in ambient illumination comprising anemissive spatial light modulator, a first polar control retarderarranged between the display polariser of the emissive spatial lightmodulator and a first additional polariser; and a reflective polariserand second polar control retarder arranged between the first additionalpolariser and a second additional polariser;

FIG. 1B is a schematic diagram illustrating in front perspective view anarrangement polarisers and polar control retarders for the embodiment ofFIG. 1A;

FIG. 1C is a schematic diagram illustrating in side perspective view theswitchable privacy display of FIG. 1A wherein the reflective polariseris omitted and the first additional polariser is a reflective polariser:

FIG. 2 is a schematic diagram illustrating in side perspective view ofan alternative structure of spatial light modulator for use in thearrangement of FIG. 1A further comprising a parallax barrier;

FIG. 3 is a schematic diagram illustrating in side perspective view ofan alternative structure of spatial light modulator for use in thearrangement of FIG. 1A comprising a transmissive spatial light modulatorand a backlight;

FIG. 4A is a schematic diagram illustrating in side perspective view aswitchable privacy display for use in ambient illumination comprisingthe transmissive spatial light modulator and backlight of FIG. 3; areflective polariser and a first polar control retarder arranged betweenthe output display polariser of the spatial light modulator and a firstadditional polariser; and a second polar control retarder arrangedbetween the input display polariser of the spatial light modulator and asecond additional polariser;

FIG. 4B is a schematic diagram illustrating in front perspective view anarrangement polarisers and polar control retarders for the embodiment ofFIG. 4A;

FIG. 5A is a schematic diagram illustrating in side perspective view aswitchable privacy display comprising the transmissive spatial lightmodulator and backlight of FIG. 3; a first polar control retarderarranged between the input display polariser of the spatial lightmodulator and a first additional polariser; and a second polar controlretarder arranged between the first additional polariser and a secondadditional polariser;

FIG. 5B is a schematic diagram illustrating in front perspective view anarrangement polarisers and polar control retarders for the embodiment ofFIG. 1A;

FIG. 6A is a schematic diagram illustrating in side perspective view astructure of a polar control retarder wherein the polar control retardercomprises a passive C plate and an active liquid crystal layercomprising a homeotropic alignment layer and a homogeneous alignmentlayer, wherein the pretilt directions of the alignment layers have acomponent in the plane of the alignment layers that are antiparallel,and the components are oriented in a first direction in the plane of thealignment layers;

FIG. 6B is a schematic diagram illustrating in side perspective view astructure of a polar control retarder wherein the polar control retardercomprises a passive C plate and an active liquid crystal layercomprising two homogenous alignment layers, wherein pretilt directionsof the alignment layers have a component in the plane of the alignmentlayers that are antiparallel;

FIG. 6C is a schematic diagram illustrating in side perspective view astructure of a polar control retarder and an active liquid crystal layercomprising two homogeneous alignment layers wherein the polar controlretarder comprises crossed A-plates;

FIG. 6D is a schematic diagram illustrating in side perspective view astructure of a polar control retarder wherein the polar control retardercomprises a passive C plate and an active liquid crystal layercomprising two homeotropic alignment layers, wherein pretilt directionsof the alignment layers have a component in the plane of the alignmentlayers that are antiparallel;

FIG. 6E is a schematic diagram illustrating in side perspective view astructure of a polar control retarder and an active liquid crystal layercomprising two homogeneous alignment layers wherein the polar controlretarder comprises crossed A-plates;

FIG. 7A is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for transmitted light from the spatiallight modulator in the public mode of operation:

FIG. 7B is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for ambient light in the public mode ofoperation;

FIG. 7C is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for transmitted light from the spatiallight modulator in a privacy mode of operation with high reflectivity ofambient light:

FIG. 7D is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for ambient light in a privacy mode ofoperation with high reflectivity of ambient light;

FIG. 8A is a schematic diagram illustrating in front perspective view anarrangement polarisers and polar control retarders for the embodiment ofFIG. 1A wherein the first polar control retarder comprises a homeotropicalignment layer and a homogeneous alignment layer and C-plate; and thesecond polar control retarder comprises two homogeneous alignment layersand crossed A-plates;

FIG. 8B is a graph illustrating a simulated polar profile of luminanceoutput of an emissive spatial light modulator:

FIG. 8C is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder of FIG. 8A arrangedbetween the first and second additional polarisers wherein the electricvector transmission directions of the polarisers are parallel;

FIG. 8D is a graph illustrating a simulated polar profile ofreflectivity of the second polar control retarder of FIG. 8A arrangedbetween a reflective polariser and the second additional polariserwherein the electric vector transmission directions of the polarisersare parallel:

FIG. 8E is a graph illustrating a simulated polar profile of the totalreflectivity comprising the reflectivity of FIG. 8D and the Fresnelreflectivity from the front surface of the display device:

FIG. 8F is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder of FIG. 8A arrangedbetween the display polariser and the first additional polariser whereinthe electric vector transmission directions of the polarisers areparallel and wherein the pretilt directions of the first polar controlretarder are parallel or anti-parallel to the pretilt directions of thesecond polar control retarder;

FIG. 8G is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders of FIG. 8A:

FIG. 8H is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement of FIG. 8A for an ambientilluminance measured in lux that is twice the head-on display luminancemeasured in nits:

FIG. 8I is a graph illustrating a simulated lateral profile for zerodegrees elevation of the visual security factor of the arrangement ofFIG. 8A for an ambient illuminance measured in lux that is twice thehead-on display luminance measured in nits:

FIG. 8J is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement of FIG. 8A for an ambientilluminance measured in lux that is twice the head-on display luminancemeasured in nits and operated in public mode;

FIG. 9A is a graph illustrating a simulated polar profile of luminanceoutput of a transmissive spatial light modulator that is illuminated bya collimated backlight;

FIG. 9B is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder;

FIG. 9C is a graph illustrating a simulated polar profile ofreflectivity of the second polar control retarder;

FIG. 9D is a graph illustrating a simulated polar profile of the totalreflectivity comprising the reflectivity of FIG. 9C and the Fresnelreflectivity from the front surface of the display device;

FIG. 9E is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder wherein the pretiltdirections of the first polar control retarder are parallel oranti-parallel to the pretilt directions of the second polar controlretarder;

FIG. 9F is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders;

FIG. 9G is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement for an ambient illuminance measuredin lux that is twice the head-on display luminance measured in nits;

FIG. 9H is a graph illustrating a simulated lateral profile for zerodegrees elevation of the visual security factor of the arrangement foran ambient illuminance measured in lux that is twice the head-on displayluminance measured in nits:

FIG. 9I is a graph illustrating a simulated polar profile of thesecurity level, S for an ambient illuminance measured in lux that istwice the head-on display luminance measured in nits and operated inpublic mode;

FIG. 10A is a graph illustrating a simulated polar profile of luminanceoutput of a transmissive spatial light modulator that is illuminated bya collimated backlight:

FIG. 10B is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder:

FIG. 10C is a graph illustrating a simulated polar profile ofreflectivity of the second polar control retarder for no reflectivepolariser;

FIG. 10D is a graph illustrating a simulated polar profile of the totalreflectivity comprising the reflectivity of FIG. 10C and the Fresnelreflectivity from the front surface of the display device for noreflective polariser:

FIG. 10E is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder wherein the pretiltdirections of the first polar control retarder are parallel oranti-parallel to the pretilt directions of the second polar controlretarder;

FIG. 10F is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders;

FIG. 10G is a graph illustrating a simulated polar profile of thesecurity level. S of the arrangement with no reflective polariser for anambient illuminance measured in lux that is twice the head-on displayluminance measured in nits;

FIG. 10H is a graph illustrating a simulated lateral profile for zerodegrees elevation of the visual security factor of the arrangement foran ambient illuminance measured in lux that is twice the head-on displayluminance measured in nits;

FIG. 11A is a graph illustrating a simulated polar profile of luminanceoutput of a transmissive spatial light modulator that is illuminated bya collimated backlight;

FIG. 11B is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder;

FIG. 11C is a graph illustrating a simulated polar profile ofreflectivity of the second polar control retarder;

FIG. 11D is a graph illustrating a simulated polar profile of the totalreflectivity comprising the reflectivity of FIG. 11C and the Fresnelreflectivity from the front surface of the display device:

FIG. 11E is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder wherein the pretiltdirections of the first polar control retarder are orthogonal to thepretilt directions of the second polar control retarder:

FIG. 11F is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders;

FIG. 11G is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement with no reflective polariser for anambient illuminance measured in lux that is twice the head-on displayluminance measured in nits for the polar profiles of FIGS. 11A-F:

FIG. 12A is a graph illustrating a simulated polar profile of luminanceoutput of an emissive spatial light modulator with the barrier structureof FIG. 2;

FIG. 12B is a graph illustrating a simulated polar profile oftransmission of the barrier structure of FIG. 2 of light from the pixelsof the emissive spatial light modulator;

FIG. 12C is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder arranged between thefirst and second additional polarisers wherein the electric vectortransmission directions of the polarisers are parallel:

FIG. 12D is a graph illustrating a simulated polar profile ofreflectivity of the second polar control retarder arranged between areflective polariser and the second additional polariser wherein theelectric vector transmission directions of the polarisers are parallel;

FIG. 12E is a graph illustrating a simulated polar profile of the totalreflectivity comprising the reflectivity and the Fresnel reflectivityfrom the front surface of the display device;

FIG. 12F is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder arranged between thedisplay polariser and the first additional polariser wherein theelectric vector transmission directions of the polarisers are parallel:

FIG. 12G is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders;

FIG. 12H is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement for an ambient illuminance measuredin lux that is twice the head-on display luminance measured in nits;

FIG. 12I is a graph illustrating a simulated polar profile of thesecurity level, S wherein the first polar control retarder is the secondpolar control retarder of FIG. 12H and the second polar control retarderis the first polar control retarder of FIG. 11H;

FIG. 12J is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement for an ambient illuminance measuredin lux that is twice the head-on display luminance measured in nits andoperated in public mode:

FIG. 13 is a schematic diagram illustrating in side perspective view aswitchable privacy display component for use with a spatial lightmodulator comprising a first polar control retarder and a firstadditional polariser, a reflective polariser; and a second polar controlretarder arranged between the first additional polariser and a secondadditional polariser;

FIG. 14 is a key for the alternative stacking arrangements of FIGS.15A-F, FIGS. 16A-F, FIGS. 17A-C and FIGS. 18A-F;

FIG. 15A, FIG. 15B, FIG. 15C, FIG. 15D, FIG. 15E, and FIG. 15F areschematic diagrams illustrating in side view alternatives for opticalcomponent stackings for a switchable privacy display wherein the firstand second polar control retarders are arranged to receive light from atransmissive spatial light modulator and backlight;

FIG. 16A, FIG. 16B, FIG. 16C, FIG. 16D, FIG. 16E, and FIG. 16F areschematic diagrams illustrating in side view alternatives for opticalcomponent stackings for a switchable privacy display wherein one of thefirst and second polar control retarders is arranged to receive lightfrom the spatial light modulator and the transmissive spatial lightmodulator is arranged to receive light from the other of the first andsecond polar control retarders and a backlight;

FIG. 17A, FIG. 17B, and FIG. 17C are schematic diagrams illustrating inside view alternatives for optical component stackings for a switchableprivacy display wherein the transmissive spatial light modulator isarranged to receive light from the first and second polar controlretarders and a backlight; and

FIG. 18A, FIG. 18B, FIG. 18C, FIG. 18D, FIG. 18E, and FIG. 18F areschematic diagrams illustrating in side view alternatives for opticalcomponent stackings for a switchable privacy display wherein the firstand second polar control retarders are arranged to receive light from anemissive spatial light modulator.

DETAILED DESCRIPTION

Terms related to optical retarders for the purposes of the presentdisclosure will now be described.

In a layer comprising a uniaxial birefringent material there is adirection governing the optical anisotropy whereas all directionsperpendicular to it (or at a given angle to it) have equivalentbirefringence.

The optical axis of an optical retarder refers to the direction ofpropagation of a light ray in the uniaxial birefringent material inwhich no birefringence is experienced. This is different from theoptical axis of an optical system which may for example be parallel to aline of symmetry or normal to a display surface along which a principalray propagates.

For light propagating in a direction orthogonal to the optical axis, theoptical axis is the slow axis when linearly polarized light with anelectric vector direction parallel to the slow axis travels at theslowest speed. The slow axis direction is the direction with the highestrefractive index at the design wavelength. Similarly the fast axisdirection is the direction with the lowest refractive index at thedesign wavelength.

For positive dielectric anisotropy uniaxial birefringent materials theslow axis direction is the extraordinary axis of the birefringentmaterial. For negative dielectric anisotropy uniaxial birefringentmaterials the fast axis direction is the extraordinary axis of thebirefringent material.

The terms half a wavelength and quarter a wavelength refer to theoperation of a retarder for a design wavelength λ₀ that may typically bebetween 500 nm and 570 nm. In the present illustrative embodimentsexemplary retardance values are provided for a wavelength of 550 nmunless otherwise specified.

The retarder provides a phase shift between two perpendicularpolarization components of the light wave incident thereon and ischaracterized by the amount of relative phase, F, that it imparts on thetwo polarization components; which is related to the birefringence Δnand the thickness d of the retarder by

Γ=2·π·Δn·d/λ ₀  eqn. 1

In eqn. 1, Δn is defined as the difference between the extraordinary andthe ordinary index of refraction, i.e.

Δn=n _(e) −n _(o)  eqn. 2

For a half-wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π.For a quarter-wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π/2.

The term half-wave retarder herein typically refers to light propagatingnormal to the retarder and normal to the spatial light modulator.

Some aspects of the propagation of light rays through a transparentretarder between a pair of polarisers will now be described.

The state of polarisation (SOP) of a light ray is described by therelative amplitude and phase shift between any two orthogonalpolarization components. Transparent retarders do not alter the relativeamplitudes of these orthogonal polarisation components but act only ontheir relative phase. Providing a net phase shift between the orthogonalpolarisation components alters the SOP whereas maintaining net relativephase preserves the SOP. In the current description, the SOP may betermed the polarisation state.

A linear SOP has a polarisation component with a non-zero amplitude andan orthogonal polarisation component which has zero amplitude.

A linear polariser transmits a unique linear SOP that has a linearpolarisation component parallel to the electric vector transmissiondirection of the linear polariser and attenuates light with a differentSOP. The term “electric vector transmission direction” refers to anon-directional axis of the polariser parallel to which the electricvector of incident light is transmitted, even though the transmitted“electric vector” always has an instantaneous direction. The term“direction” is commonly used to describe this axis.

Absorbing polarisers are polarisers that absorb one polarisationcomponent of incident light and transmit a second orthogonalpolarisation component. Examples of absorbing linear polarisers aredichroic polarisers.

Reflective polarisers are polarisers that reflect one polarisationcomponent of incident light and transmit a second orthogonalpolarisation component. Examples of reflective polarisers that arelinear polarisers are multilayer polymeric film stacks such as DBEF™ orAPF™ from 3M Corporation, or wire grid polarisers such as ProFlux™ fromMoxtek. Reflective linear polarisers may further comprise cholestericreflective materials and a quarter waveplate arranged in series.

A retarder arranged between a linear polariser and a parallel linearanalysing polariser that introduces no relative net phase shift providesfull transmission of the light other than residual absorption within thelinear polariser.

A retarder that provides a relative net phase shift between orthogonalpolarisation components changes the SOP and provides attenuation at theanalysing polariser.

In the present disclosure an ‘A-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisparallel to the plane of the layer.

A ‘positive A-plate’ refers to positively birefringent A-plates, i.e.A-plates with a positive Δn.

In the present disclosure a ‘C-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisperpendicular to the plane of the layer. A ‘positive C-plate’ refers topositively birefringent C-plate, i.e. a C-plate with a positive Δn. A‘negative C-plate’ refers to a negatively birefringent C-plate, i.e. aC-plate with a negative Δn.

‘O-plate’ refers to an optical retarder utilizing a layer ofbirefringent material with its optical axis having a component parallelto the plane of the layer and a component perpendicular to the plane ofthe layer. A ‘positive O-plate’ refers to positively birefringentO-plates, i.e. O-plates with a positive Δn.

Achromatic retarders may be provided wherein the material of theretarder is provided with a retardance λn, d that varies with wavelengthλ as

Δn·d/λ=κ  eqn. 3

where κ is substantially a constant.

Examples of suitable materials include modified polycarbonates fromTeijin Films. Achromatic retarders may be provided in the presentembodiments to advantageously minimise color changes between polarangular viewing directions which have low luminance reduction and polarangular viewing directions which have increased luminance reductions aswill be described below.

Various other terms used in the present disclosure related to retardersand to liquid crystals will now be described.

A liquid crystal cell has a retardance given by λn, d where Δn is thebirefringence of the liquid crystal material in the liquid crystal celland d is the thickness of the liquid crystal cell, independent of thealignment of the liquid crystal material in the liquid crystal cell.

Homogeneous alignment refers to the alignment of liquid crystals inswitchable liquid crystal displays where molecules align substantiallyparallel to a substrate. Homogeneous alignment is sometimes referred toas planar alignment. Homogeneous alignment may typically be providedwith a small pre-tilt such as 2 degrees, so that the molecules at thesurfaces of the alignment layers of the liquid crystal cell are slightlyinclined as will be described below. Pretilt is arranged to minimisedegeneracies in switching of cells.

In the present disclosure, homeotropic alignment is the state in whichrod-like liquid crystalline molecules align substantiallyperpendicularly to the substrate. In discotic liquid crystalshomeotropic alignment is defined as the state in which an axis of thecolumn structure, which is formed by disc-like liquid crystallinemolecules, aligns perpendicularly to a surface. In homeotropicalignment, pretilt is the tilt angle of the molecules that are close tothe alignment layer and is typically close to 90 degrees and for examplemay be 88 degrees.

In a twisted liquid crystal layer a twisted configuration (also known asa helical structure or helix) of nematic liquid crystal molecules isprovided. The twist may be achieved by means of a non-parallel alignmentof alignment layers. Further, cholesteric dopants may be added to theliquid crystal material to break degeneracy of the twist direction(clockwise or anti-clockwise) and to further control the pitch of thetwist in the relaxed (typically undriven) state. A supertwisted liquidcrystal layer has a twist of greater than 180 degrees. A twisted nematiclayer used in spatial light modulators typically has a twist of 90degrees.

Liquid crystal molecules with positive dielectric anisotropy areswitched from a homogeneous alignment (such as an A-plate retarderorientation) to a homeotropic alignment (such as a C-plate or O-plateretarder orientation) by means of an applied electric field.

Liquid crystal molecules with negative dielectric anisotropy areswitched from a homeotropic alignment (such as a C-plate or O-plateretarder orientation) to a homogeneous alignment (such as an A-plateretarder orientation) by means of an applied electric field.

Rod-like molecules have a positive birefringence so that n_(e)>n_(o) asdescribed in eqn. 2. Discotic molecules have negative birefringence sothat n_(e)<n_(o).

Positive retarders such as A-plates, positive O-plates and positiveC-plates may typically be provided by stretched films or rod-like liquidcrystal molecules. Negative retarders such as negative C-plates may beprovided by stretched films or discotic like liquid crystal molecules.

Parallel liquid crystal cell alignment refers to the alignment directionof homogeneous alignment layers being parallel or more typicallyantiparallel. In the case of pre-tilted homeotropic alignment, thealignment layers may have components that are substantially parallel orantiparallel. Hybrid aligned liquid crystal cells may have onehomogeneous alignment layer and one homeotropic alignment layer. Twistedliquid crystal cells may be provided by alignment layers that do nothave parallel alignment, for example oriented at 90 degrees to eachother.

Transmissive spatial light modulators may further comprise retardersbetween the input display polariser and the output display polariser forexample as disclosed in U.S. Pat. No. 8,237,876, which is hereinincorporated by reference in its entirety. Such retarders (not shown)are in a different place to the passive retarders of the presentembodiments. Such retarders compensate for contrast degradations foroff-axis viewing locations, which is a different effect to the luminancereduction for off-axis viewing positions of the present embodiments.

A private mode of operation of a display is one in which an observersees a low contrast sensitivity such that an image is not clearlyvisible. Contrast sensitivity is a measure of the ability to discernbetween luminances of different levels in a static image. Inversecontrast sensitivity may be used as a measure of visual security, inthat a high visual security level (VSL) corresponds to low imagevisibility.

For a privacy display providing an image to an observer, visual securitymay be given as:

V=(Y+R)/(Y−K)  eqn. 4

where V is the visual security level (VSL), Y is the luminance of thewhite state of the display at a snooper viewing angle, K is theluminance of the black state of the display at the snooper viewing angleand R is the luminance of reflected light from the display.

Panel contrast ratio is given as:

C=Y/K  eqn. 5

so the visual security level may be further given as:

V=(P·Y _(max) ++I·ρ/π)/(P·(Y _(max) −Y _(max) /C))  eqn. 6

where: Y_(max) is the maximum luminance of the display; P is theoff-axis relative luminance typically defined as the ratio of luminanceat the snooper angle to the maximum luminance Y_(max); C is the imagecontrast ratio; ρ is the surface reflectivity; and I is the illuminance.The units of Y_(max) are the units of I divided by solid angle in unitsof steradian.

The luminance of a display varies with angle and so the maximumluminance of the display Y_(max) occurs at a particular angle thatdepends on the configuration of the display.

In many displays, the maximum luminance Y_(max) occurs head-on, i.e.normal to the display. Any display device disclosed herein may bearranged to have a maximum luminance Y_(max) that occurs head-on, inwhich case references to the maximum luminance of the display deviceY_(max) may be replaced by references to the luminance normal to thedisplay device.

Alternatively, any display described herein may be arranged to have amaximum luminance Y_(max) that occurs at a polar angle to the normal tothe display device that is greater than 0°. By way of example, themaximum luminance Y_(max) may occur may at a non-zero polar angle and atan azimuth angle that has for example zero lateral angle so that themaximum luminance is for an on-axis user that is looking down on to thedisplay device. The polar angle may for example be 10 degrees and theazimuthal angle may be the northerly direction (90 degreesanti-clockwise from easterly direction). The viewer may thereforedesirably see a high luminance at typical non-normal viewing angles.

The off-axis relative luminance, P is sometimes referred to as theprivacy level. However, such privacy level P describes relativeluminance of a display at a given polar angle compared to head-onluminance, and in fact is not a measure of privacy appearance.

The illuminance, I is the luminous flux per unit area that is incidenton the display and reflected from the display towards the observerlocation. For Lambertian illuminance, and for displays with a Lambertianfront diffuser illuminance I is invariant with polar and azimuthalangles. For arrangements with a display with non-Lambertian frontdiffusion arranged in an environment with directional (non-Lambertian)ambient light, illuminance I varies with polar and azimuthal angle ofobservation.

Thus in a perfectly dark environment, a high contrast display has VSL ofapproximately 1.0. As ambient illuminance increases, the perceived imagecontrast degrades. VSL increases and a private image is perceived.

For typical liquid crystal displays the panel contrast C is above 100:1for almost all viewing angles. allowing the visual security level to beapproximated to:

V=1+I·ρ/(π·P·Y _(max))  eqn. 7

In the present embodiments, in addition to the exemplary definition ofeqn. 4, other measurements of visual security level, V may be provided,for example to include the effect on image visibility to a snooper ofsnooper location, image contrast, image colour and white point andsubtended image feature size. Thus the visual security level may be ameasure of the degree of privacy of the display but may not berestricted to the parameter V.

The perceptual image security may be determined from the logarithmicresponse of the eye, such that

S=log₁₀(V)  eqn. 8

Desirable limits for S were determined in the following manner. In afirst step a privacy display device was provided. Measurements of thevariation of privacy level. P(θ) of the display device with polarviewing angle and variation of reflectivity ρ(θ) of the display devicewith polar viewing angle were made using photopic measurement equipment.A light source such as a substantially uniform luminance light box wasarranged to provide illumination from an illuminated region that wasarranged to illuminate the privacy display device along an incidentdirection for reflection to a viewer positions at a polar angle ofgreater than 0° to the normal to the display device. The variation I(θ)of illuminance of a substantially Lambertian emitting lightbox withpolar viewing angle was determined by and measuring the variation ofrecorded reflective luminance with polar viewing angle taking intoaccount the variation of reflectivity ρ(θ). The measurements of P(θ),r(θ) and I(θ) were used to determine the variation of Security FactorS(θ) with polar viewing angle along the zero elevation axis.

In a second step a series of high contrast images were provided on theprivacy display including (i) small text images with maximum font height3 mm, (ii) large text images with maximum font height 30 mm and (iii)moving images.

In a third step each observer (with eyesight correction for viewing at1000 mm where appropriate) viewed each of the images from a distance of1000 mm, and adjusted their polar angle of viewing at zero elevationuntil image invisibility was achieved for one eye from a position nearon the display at or close to the centre-line of the display. The polarlocation of the observer's eye was recorded. From the relationship S(θ),the security factor at said polar location was determined. Themeasurement was repeated for the different images, for various displayluminance Y_(max), different lightbox illuminance I(θ=0), for differentbackground lighting conditions and for different observers.

From the above measurements S<1.0 provides low or no visual security,1.0≤S<1.5 provides visual security that is dependent on the contrast,spatial frequency and temporal frequency of image content, 1.5≤S<1.8provides acceptable image invisibility (that is no image contrast isobservable) for most images and most observers and S≥1.8 provides fullimage invisibility, independent of image content for all observers.

In practical display devices, this means that it is desirable to providea value of S for an off-axis viewer who is a snooper that meets therelationship S≥S_(min), where: S_(min) has a value of 1.0 or more toachieve the effect that the off-axis viewer cannot perceive thedisplayed image; S_(min) has a value of 1.5 or more to achieve theeffect that the displayed image is invisible, i.e. the viewer cannotperceive even that an image is being displayed, for most images and mostobservers; or S_(min) has a value of 1.8 or more to achieve the effectthat the displayed image is invisible independent of image content forall observers.

In comparison to privacy displays, desirably wide angle displays areeasily observed in standard ambient illuminance conditions. One measureof image visibility is given by the contrast sensitivity such as theMichelson contrast which is given by:

M=(I _(max) −I _(min))/(I _(max) +I _(min))  eqn. 9

and so:

M=((Y+R)−(K+R))/((Y+R)+(K+R))=(Y−K)/(Y+K+2·R)  eqn. 10

Thus the visual security level (VSL), V is equivalent (but not identicalto) 1/M. In the present discussion, for a given off-axis relativeluminance, P the wide angle image visibility, W is approximated as

W=1/V=1/(1+I·ρ/(π·P·Y _(max)))  eqn. 11

The above discussion focusses on reducing visibility of the displayedimage to an off-axis viewer who is a snooper, but similar considerationsapply to visibility of the displayed image to the intended user of thedisplay device who is typically on-axis. In this case, decrease of thelevel of the visual security level (VSL) V corresponds to an increase inthe visibility of the image to the viewer. During observation S<0.1 mayprovide acceptable visibility of the displayed image. In practicaldisplay devices, this means that it is desirable to provide a value of Sfor an on-axis viewer who is the intended user of the display devicethat meets the relationship S≤S_(max), where S_(max) has a value of 0.1.

In the present discussion the colour variation Δε of an output colour(u_(w)′+Δu′, v_(w)′+Δv′) from a desirable white point (u_(w)′, v_(w)′)may be determined by the CIELUV colour difference metric, assuming atypical display spectral illuminant and is given by:

Δε=(Δu′ ² +Δv′ ²)^(1/2)  eqn. 12

The structure and operation of various directional display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

FIG. 1A is a schematic diagram illustrating in side perspective view aswitchable privacy display device 100 for use in ambient illumination604 comprising an emissive spatial light modulator 48, a first polarcontrol retarder 300A arranged between the display polariser 218 of theemissive spatial light modulator 48 and a first additional polariser318A; and a reflective polariser 302 and second polar control retarder300B arranged between the first additional polariser 318A and a secondadditional polariser 318B; and FIG. 1B is a schematic diagramillustrating in front perspective view an arrangement polarisers andpolar control retarders for the embodiment of FIG. 1A.

The display device 100 comprises a spatial light modulator 48; whereinthe spatial light modulator 48 comprises an emissive spatial lightmodulator 48 arranged to output light, the display polariser 218 is anoutput display polariser arranged on the output side of the emissivespatial light modulator 48, the display polariser 218 being a linearpolariser.

A quarter waveplate 202 is arranged between the display polariser 218and the pixel plane 214 to reduce frontal reflections from the pixelplane 214. Substrates 212, 216 are arranged to provide support of thepixel plane 214.

A first additional polariser 318A is arranged on the same side of thespatial light modulator 48 as the display polariser 218, the firstadditional polariser 318 being a linear polariser. The first additionalpolariser 318A is an absorbing polariser such as an iodine polariser onstretched PVA.

At least one first polar control retarder 300A is arranged between thefirst additional polariser 318A and the display polariser 218.

The display device 100 further comprises a second additional polariser318B, the second additional polariser being a linear polariser; and atleast one second polar control retarder 300B. The second additionalpolariser 318B is arranged on the output side of the spatial lightmodulator 48 outside the first additional polariser 318A, and the atleast one second polar control retarder 318B is arranged between thefirst additional polariser 318A and the second additional polariser 31B.

Said display polariser 218 is an output display polariser arranged onthe output side of the spatial light modulator 48, and the displaydevice further comprises a reflective polariser 302 arranged between thefirst additional polariser 318A and at least one second polar controlretarder 300B, the reflective polariser being a linear polariser.

Each of the at least one first polar control retarder 300A and the atleast one second polar control retarder 300B comprises a respectiveswitchable liquid crystal retarder 301A, 301B comprising a layer ofliquid crystal material 314A, 314B, arranged between transparentsubstrates 312A. 312B and 316A, 316B respectively.

Each of the at least one first polar control retarder 300A and at leastone second polar control retarder 300B further comprises at least onepassive compensation retarder 330A, 330B respectively.

In an alternative embodiment (not shown), reflective polariser 302 maybe omitted.

FIG. 1C is a schematic diagram illustrating in side perspective view theswitchable privacy display device 100 of FIG. 1A wherein the reflectivepolariser 302 is omitted and the first additional polariser 318A is areflective polariser. Advantageously thickness and cost may be reduced.Features of the embodiment of FIG. 1A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

The spatial light modulator 48 may take any suitable form. Some possiblealternatives are as follows.

FIG. 2 is a schematic diagram illustrating in side perspective view ofan alternative for the structure of emissive spatial light modulator 48for use in the arrangement of FIG. 1A. The spatial light modulator 48further comprising a parallax barrier 70) comprising light absorbingregion 704 and apertures 702 that are aligned with the pixels 220, 222,224 of the pixel plane 214. The parallax barrier 700 is separated bydistance d from the pixel plane 214 and is aligned to the pixels so thatthe pixels have high luminance on axis and reduced luminance off-axis.

In operation the parallax barrier 700 is arranged to providetransmission of light ray 440 from pixel 224 in the normal direction tothe spatial light modulator 48, and the aligned aperture 702 is arrangedwith an aperture size to provide high transmission. By comparison lightrays 442 that are inclined at a non-zero polar angle, may be absorbed inthe absorbing region 704. The separation d is provided to achieve aminimum transmission at a desirable polar angle in at least oneazimuthal direction. Advantageously off-axis luminance is reduced,achieving increased security factor.

Further, reflectivity of the pixel plane may be reduced as incidentambient light is absorbed at the absorbing region 704. Quarter waveplate202 of FIGS. 1A-1B may be omitted achieving reduced cost and complexity.

Features of the embodiment of FIG. 2 not discussed in further detail maybe assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Another alternative for the emissive spatial light modulator 48 for usein the arrangement of FIGS. 1A-B will now be described.

FIG. 3 is a schematic diagram illustrating in side perspective view ofan alternative structure of spatial light modulator for use in thearrangement of FIG. 1A comprising a transmissive spatial light modulator48 and a backlight 20 arranged to output light. The spatial lightmodulator 48 comprises a transmissive spatial light modulator arrangedto receive output light from the backlight 20. Features of theembodiment of FIG. 1A not discussed in further detail may be assumed tocorrespond to the features with equivalent reference numerals asdiscussed above, including any potential variations in the features.

The backlight may comprise a light guide plate (LGP) 1, light extractionlayers 5 and rear reflector 3. The light extraction layers may comprisediffusers, light turning films or prism films. Light may be providedfrom an array of light sources such as LEDs 15 arranged at the edge ofthe LGP 1.

The output may be provide a wide angle luminance profile such asachieved using crossed BEF™ films from 3M corporation and may have afull width half maximum of greater than 50 degrees. The output mayprovide a narrow angle profile, such backlights may be termed collimatedbacklights and have a full width half maximum luminance of less than 50degrees, for example 30 degrees. Examples of collimated backlights areillustrated in U.S. Pat. No. 10,935,714, which is herein incorporated byreference in its entirety. The backlight may comprise other types ofstructure including mini-LED arrays and known light distribution opticsto achieve desirable uniformity. The backlight 20 may be furtherprovided with a micro-louvre array arranged to reduce off-axis luminanceoutput from the backlight 20. Advantageously security factor, S may beimproved in comparison to wide angle backlights.

Alternative arrangements of polar control retarders and additionalpolarisers will now be described for display devices 100 comprisingbacklights 20.

FIG. 4A is a schematic diagram illustrating in side perspective view aswitchable privacy display device 100 for use in ambient illumination604 comprising the transmissive spatial light modulator 48 and backlight20 of FIG. 3; and FIG. 4B is a schematic diagram illustrating in frontperspective view an arrangement polarisers and polar control retardersfor the embodiment of FIG. 4A.

The display device 100 further comprises an input display polariser 210arranged on the input side of the spatial light modulator 48, and thedisplay device 100 further comprises an output display polariser 218arranged on the output side of the spatial light modulator 48, the firstadditional polariser 318A is arranged on the input side of the spatiallight modulator 48 and the first polar control retarder 300A is arrangedbetween the first additional polariser 318A and the input displaypolariser 210. The second additional polariser 318B is arranged on theoutput side of the spatial light modulator 48, and the at least onesecond polar control retarder 300B is arranged between the secondadditional polariser 318B and the output display polariser 218.

Second polar control retarder 300B is arranged between the input displaypolariser 210 of the spatial light modulator 48 and a second additionalpolariser 318B. Reflective polariser 302 is arranged between the outputdisplay polariser 218 and the second polar control retarder 318B. In analternative embodiment (not shown), reflective polariser 302 may beomitted.

Advantageously the separation of the output of the second additionalpolariser 318B to the pixel plane 214 is reduced in comparison to thearrangements of FIGS. 1A-B. Contrast of the image seen may be increaseddue to the reduced number of layers. An air gap may be provided betweenthe input polariser 210 and second polar retarder 300B, advantageouslyreducing assembly cost and complexity.

In the present embodiments, said one of the at least one first polarcontrol retarder 300A and the at least one second polar control retarder300B is the at least one second polar control retarder and said other ofthe at least one first polar control retarder and the at least onesecond polar control retarder is the at least one first polar controlretarder.

Features of the embodiment of FIG. 4A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

The embodiments of FIGS. 1A-B and FIG. 4A may alternatively be providedwith no reflective polariser 302. Advantageously frontal reflections arereduced in privacy mode in environments where increased reflection isconsidered undesirable. Another alternative arrangement with noreflective polariser will now be described.

FIG. 5A is a schematic diagram illustrating in side perspective view aswitchable privacy display device 100 comprising the transmissivespatial light modulator 48 and backlight 20 of FIG. 3; and FIG. 5B is aschematic diagram illustrating in front perspective view an arrangementpolarisers and polar control retarders for the embodiment of FIG. 5A.

A first polar control retarder 300A is arranged between the inputdisplay polariser 210 of the spatial light modulator 48 and a firstadditional polariser 318A; and a second polar control retarder 300B isarranged between the first additional polariser 318A and a secondadditional polariser 318B.

Reflective polariser 302 is omitted. In some environments such ascertain automotive environments, reflective operation may be undesirableand front of display reflectivity may be reduced. Further cost may bereduced.

In comparison to the arrangements of FIG. 1 and FIG. 4A the outputdisplay polariser 218 is the display device 100 output polariser.Advantageously diffusers may be arranged on the polariser 218 to provideincreased image haze with reduced image blurring. Air gaps may beprovided between the spatial light modulator input polariser 210 andplural retarders 300A, 300B. Advantageously image contrast is notdegraded and assembly cost and complexity is reduced.

Features of the embodiment of FIG. 5A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Arrangements of liquid crystal alignment for use in the retarders 300A,300B of FIG. 1, FIG. 4A and FIG. 5A will now be described.

FIG. 6A is a schematic diagram illustrating in side perspective view astructure of a polar control retarder 300 wherein the polar controlretarder 300 comprises a passive C plate retarder 330 and an activeliquid crystal layer 314.

Electrodes 413, 415 are arranged to apply a voltage from driver 350across the liquid crystal material 421 in the layer 314. In a firstdriven state the liquid crystal molecules are arranged to provide nophase modification to input polarisation state in a normal direction tothe polar control retarder and modified phase to an input polarisationstate in directions at an angle to the normal direction to the polarcontrol retarder 300. Such a driven state may be provided for privacymode operation.

In a second driven state the liquid crystal molecules are arranged toprovide no phase modification to input polarisation state in a normaldirection to the polar control retarder and modified phase to an inputpolarisation state in directions at an angle to the normal direction tothe polar control retarder 300. Such a driven state may be provided forpublic (or share) mode operation.

Two surface alignment layers are disposed adjacent to the layer ofliquid crystal material and on opposite sides thereof. One of thesurface alignment layers being arranged to provide homogenous alignmentin the adjacent liquid crystal material and the other of the surfacealignment layers being arranged to provide homeotropic alignment in theadjacent liquid crystal material. The alignment layers thus comprise ahomeotropic alignment layer 417A and a homogeneous alignment layer 417B.

The pretilt directions 419A, 419B of the alignment layers have acomponent in the plane of the alignment layers 417A, 417B that areantiparallel. The pretilt directions 419A, 419B refer to the alignmentof the liquid crystal molecules 421 that are adjacent to said layers.The components 419Ay and 419By are the in-plane components and areanti-parallel to each other. Component 419Az at the homeotropicalignment layer 417A is much greater than component 419Ay whilecomponent 419Bz at the homogeneous alignment layer 417B is much smallerthan component 41By. The pretilt angle is the angle between thedirections 419A and 419Ay, and between directions 419B and 419Byrespectively.

The components 419Ay, 419By are oriented in a first direction in theplane of the alignment layers, that is parallel to the y-axis.

In the present illustrative embodiments and as illustrated in FIG. 6A,the at least one passive compensation retarder 330 of said one of the atleast one first polar control retarder 330A and the at least one secondpolar control retarder 330B is arranged on the same side of theswitchable liquid crystal retarder 314 as the surface alignment layers417A that is arranged to provide homeotropic alignment in the adjacentliquid crystal material 421. The at least one second polar controlretarder 330 comprises a passive uniaxial retarder having its opticalaxis perpendicular to the plane of the retarder 330, that is a C-plate.

In alternative embodiments, the passive retarder 330 may comprise‘crossed’ A-plates. In the present disclosure crossed A-plates refers toa pair of passive uniaxial retarders having optical axes in the plane ofthe retarders that are crossed, as illustrated by retarders 330A, 330Bin FIG. 6C.

FIG. 6B is a schematic diagram illustrating in side perspective view astructure of a polar control retarder wherein the polar control retardercomprises a passive C plate and an active liquid crystal layercomprising two homogeneous alignment layers, wherein pretilt directionsof the alignment layers have a component in the plane of the alignmentlayers that are antiparallel.

FIG. 6C is a schematic diagram illustrating in side perspective view astructure of a polar control retarder and an active liquid crystal layercomprising two homogeneous alignment layers wherein the polar controlretarder comprises crossed A-plates.

FIG. 6D is a schematic diagram illustrating in side perspective view astructure of a polar control retarder wherein the polar control retardercomprises a passive C plate and an active liquid crystal layercomprising two homeotropic alignment layers, wherein pretilt directionsof the alignment layers have a component in the plane of the alignmentlayers that are antiparallel.

FIG. 6E is a schematic diagram illustrating in side perspective view astructure of a polar control retarder and an active liquid crystal layercomprising two homogeneous alignment layers wherein the polar controlretarder comprises crossed A-plates. Advantageously higher retardancesof the passive retarder 330A, 330B may be provided at lower cost incomparison to a C-plate 330 embodiment of FIG. 6D.

In the present embodiments one of the polar control retarders 300A, 300Bmay comprise the polar control retarder 300 of FIG. 6A and the other ofthe polar control retarders 300A, 300B may comprise one of the polarcontrol retarders 300 of FIGS. 6B-E. The at least one passivecompensation retarder of said other of the at least one first polarcontrol retarder and the at least one second polar control retardercomprises either: a passive uniaxial retarder having its optical axisperpendicular to the plane of the retarder; or a pair of passiveuniaxial retarders having optical axes in the plane of the retardersthat are crossed.

In respect of said other (with alignment layers that are of the sametype) of the at least one first polar control retarder and the at leastone second polar control retarder, both of the surface alignment layersmay be arranged to provide homeotropic alignment in the adjacent liquidcrystal material. Advantageously reduced voltage and power consumptionmay be provided in public mode in comparison to arrangements in whichboth alignment layers have homogeneous alignment.

In respect of said other (with alignment layers that are of the sametype) of the at least one first polar control retarder and the at leastone second polar control retarder, both of the surface alignment layersmay be arranged to provide homogeneous alignment in the adjacent liquidcrystal material. Advantageously the visibility of defects to appliedpressure may be reduced in comparison to arrangements in which bothalignment layers have homeotropic alignment.

Features of the embodiment of FIGS. 6A-C not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Referring to the arrangements of FIGS. 1A-B, FIGS. 4A-B and FIGS. 5A-B,in respect of one of the at least one first polar control retarder 300Aand the at least one second polar control retarder 300B, one of thesurface alignment layers 417A or 417B is arranged to provide homogenousalignment in the adjacent liquid crystal material and the other of thesurface alignment layers 417B or 417A is arranged to provide homeotropicalignment in the adjacent liquid crystal material 421, and in respect ofthe other of the at least one first polar control retarder 300A and theat least one second polar control retarder 300B, both of the surfacealignment layers 417A, 417B are arranged to provide homogenous alignmentin the adjacent liquid crystal material or both of the surface alignmentlayers 417A, 417B are arranged to provide homeotropic alignment in theadjacent liquid crystal material.

Operation of polar control retarders between parallel polarisers isdescribed further in U.S. Pat. No. 10,126,575 and in U.S. Patent Publ.No. 2019-0086706 (Atty. Ref. No. 412101), both of which are hereinincorporated by reference in their entireties. The operation of theplural polar control retarders of the present embodiments in a publicmode of operation will now be described.

FIG. 7A is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for transmitted light from the spatiallight modulator in the public mode of operation. In the embodiments thatwill be described below, light ray 400 that is normal to the display (orin a head-on direction) is transmitted by the display polariser 219 witha polarisation state 360 that is unmodified by the polar controlretarders 300A, 300B and polarisers 318A, 302 and 318B. Such light istransmitted with high luminance.

In public mode, rays 402 with a non-zero polar angle to the normaldirection are also transmitted with the same polarisation state 360 thatis substantially not modified by the polar control retarders 300A, 300Band polarisers 318A, 302 and 318B. The polar profile of luminance fromthe spatial light modulator may be substantially unmodified.Advantageously the display may be visible from a wide range of polarviewing positions and viewable by multiple display users.

FIG. 7B is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for incident ambient light 604 in thepublic mode of operation. Light rays 404, 406 are incident on thedisplay device 100 with substantially unpolarised state 370. Thepolariser 318B provides a polarisation state 360 that is incident on thefirst polar control retarder and is substantially unmodified for head-onray 404 and off-axis ray 406. Thus the light rays are substantially notreflected by the display are absorbed in the spatial light modulator 48and backlight 20 if present. The display reflectivity is maintained at alow level for a wide range of viewing directions and advantageously ahigh contrast image is seen by multiple display users.

The operation of the polar control retarders in a private mode ofoperation will now be described.

FIG. 7C is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for transmitted light from the spatiallight modulator in a privacy mode of operation with high reflectivity ofambient light. Head-on light ray 400 has a polarisation state 360 thatis substantially unmodified by polar control retarders 300A, 300B. Bycomparison, off-axis light ray 402 has an output from the first polarcontrol retarder that has an imparted phase difference to provide ingeneral an elliptical state 362A. On incidence with first additionalpolariser 318A the luminance of the ray 402 is reduced with output state360. Said light ray 402 is transmitted through reflective polariser 302with small loss and is incident on the second polar control retarder300B at which further phase modulation is provided and an outputpolarisation state 362B is achieved. Such state 362B is at least in partabsorbed by second additional polariser 318B. Ray 402 is thustransmitted at off-axis polar locations with reduced luminance comparedto the ray 402 in FIG. 7A.

FIG. 7D is a schematic diagram illustrating in side view the operationof the arrangement of FIG. 1A for ambient light in a privacy mode ofoperation with high reflectivity of ambient light. Head-on incidentambient light ray 404 is transmitted with substantial reflection fromthe reflective polariser 302.

By comparison light ray 406 undergoes a phase modulation at the polarcontrol retarder 300B such that state 364 illuminates the reflectivepolariser. The resolved polarisation state 366 that is orthogonal to theelectric vector transmission direction 303 of the reflective polariser302 is reflected and is passed through the polar retarder such thatpolarisation state 368 is incident on to the second additionalpolariser. The component of the state 368 that is parallel to theelectric vector transmission direction of the polariser 318B is thustransmitted. To an off-axis observer, the display appears to haveincreased reflectivity. Said increased reflectivity advantageouslyachieves increased security factor, S as described above.

Features of the embodiments of FIGS. 7A-D not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

The operation of FIGS. 7A-D is common to the arrangements of FIGS. 1A-Band FIG. 4A.

The embodiments of FIGS. 1A-B and FIG. 4A may further be provided withno reflective polariser 302. In such non-reflective structures and inFIG. 5A, the operation of FIGS. 7A and 7C only apply, that is thereflectivity does not increase in embodiments in which the reflectivepolariser 302 is omitted. Advantageously displays may be provided withlow reflectivity in privacy mode of operation. For example displays foruse in bright ambient environments such as for daytime operation inautomotive vehicles may not provide a reflectivity enhancement inprivacy mode.

It may be desirable to provide a switchable privacy display with narrowangle privacy switch-on and with a wide angle public mode.

FIG. 8A is a schematic diagram illustrating in front perspective view anarrangement polarisers and polar control retarders for the embodiment ofFIG. 1A wherein the first polar control retarder 300A comprises ahomeotropic alignment 419AA and a homogeneous alignment layer 419AB andC-plate 330A: and the second polar control retarder 300B comprises twohomogeneous alignment layers 419BA, 419BB and the passive controlretarder comprises crossed A-plates 330BA, 330BB.

Features of the embodiment of FIG. 8A not discussed in further detailmay be assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

In the present embodiments, liquid crystal retarders 301 with twohomeotropic alignment layers are preferably provided with passiveretarder 330 comprising a negative C plate. In alternative embodimentssuch retarders may also be provided by crossed A-plates, however suchembodiments are not described in further detail herein.

In the present embodiments, liquid crystal retarders 301 with twohomogeneous alignment layers are preferably provided with passiveretarder 330 comprising crossed A-plates or at least one negative Cplate.

In the present embodiments, liquid crystal retarders 301 with onehomogeneous alignment layer and one homeotropic alignment layer arepreferably provided with passive retarder 330 comprising crossed atleast one negative C plate. Preferably the negative C plate is on thesame side of the liquid crystal layer 314 as the homeotropic alignmentlayer, that is the homeotropic alignment layer 417A is between theliquid crystal layer 314 and the passive retarder 330. In alternativeembodiments such retarders may also be provided by crossed A-plates,however such embodiments are not described in further detail herein.

The illustrative embodiment of TABLE A will now be discussed withreference to FIGS. 8B-J.

TABLE 1A Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 0 700 nm 417BAHomogeneous 2 180 {550 nm~800 nm} (450 nm~900 nm) 330BA Positive +570 nmA-plate {+400~625 nm} @ 45° (+250 nm~+800 nm) 330BB Positive +570 nmA-plate {+400~625 nm} @ 135° (+250 nm~+800 nm) 300A 417AB Homogeneous 2180 1250 nm 417AA Homeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm)330A Negative −1000 nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm)

In the present embodiments, the switchable liquid crystal retarder 314of said one (with a homeotropic alignment layer and a homogeneousalignment layer) of the at least one first polar control retarder andthe at least one second polar control retarder that is arranged toprovide homogenous alignment in the adjacent liquid crystal material andthe other of the surface alignment layers is arranged to providehomeotropic alignment in the adjacent liquid crystal material, is thefirst polar control retarder 300A. Said retarder has a liquid crystalretarder 314 that has a retardance for light of a wavelength of 550 nmhaving a first retardance value in a range from 700 nm to 2500 nm,preferably in a range from 850 nm to 1800 mm. Said one 300A of the atleast one first polar control retarder and the at least one second polarcontrol retarder further comprises at least one passive compensationretarder.

As disclosed in TABLE 1A, the passive uniaxial retarder 330A has aretardance for light of a wavelength of 550 nm in a range from −400 nmto −2100 nm, preferably in a range from −700 nm to −1700 nm.

The switchable liquid crystal retarder 314A of said one (with ahomeotropic alignment layer and a homogeneous alignment layer) firstpolar control retarder has a retardance for light of a wavelength of 550nm having a first retardance value and the switchable liquid crystalretarder of said other (with two homogeneous alignment layers) of thesecond polar control retarder 300B has a retardance for light of awavelength of 550 nm has a second retardance value, the first retardancevalue being greater than the second retardance value.

The magnitude of the difference between half the first retardance valueand the second retardance value is at most 400 nm and preferably at most200 nm.

FIG. 8B is a graph illustrating a simulated polar profile of luminanceoutput of an emissive spatial light modulator. As illustrated in FIG.1A, the spatial light modulator 48 comprises an emissive spatial lightmodulator arranged to output light, the display polariser is an outputdisplay polariser 218 arranged on the output side of the emissivespatial light modulator 48, the second additional polariser 318B isarranged on the output side of the spatial light modulator 48 outsidethe first additional polariser 318A, and the at least one second polarcontrol retarder 300B is arranged between the first additional polariser318A and the second additional polariser 318B. FIG. 8B illustrates thatthe emissive spatial light modulator 48 has an output luminance profilehaving a full width half maximum that is at least 40 degrees, preferablyat least 50 degrees.

In an alternative arrangement such as illustrated in FIG. 3 the displaydevice may further comprises a backlight 20 arranged to output light,and the spatial light modulator 48 comprises a transmissive spatiallight modulator arranged to receive output light from the backlight 20.The backlight may have an output luminance profile having a full widthhalf maximum that is at least 40 degrees, preferably at least 50degrees.

FIG. 8C is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder of FIG. 8A and TABLE1A arranged between the first and second additional polarisers whereinthe electric vector transmission directions of the polarisers areparallel; FIG. 8D is a graph illustrating a simulated polar profile ofreflectivity of the second polar control retarder of FIG. 8A arrangedbetween a reflective polariser and the second additional polariserwherein the electric vector transmission directions of the polarisersare parallel; and FIG. 8E is a graph illustrating a simulated polarprofile of the total reflectivity comprising the reflectivity of FIG. 8Dand the Fresnel reflectivity from the front surface of the displaydevice.

FIG. 8F is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder of FIG. 8A and TABLE 1Aarranged between the display polariser and the first additionalpolariser wherein the electric vector transmission directions of thepolarisers are parallel. The pretilt directions of the first polarcontrol retarder 300A are parallel or anti-parallel to the pretiltdirections of the second polar control retarder 300B.

FIG. 8G is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders of FIG. 8A and TABLE 1A for thepolar profiles of FIGS. 8B-F.

As described above, the security performance of the display can bedetermined using the polar variation of visual security level, S withpolar viewing angle.

FIG. 8H is a graph illustrating a simulated polar profile of thesecurity level. S of the arrangement of FIG. 8A and TABLE 1A for anambient illuminance measured in lux that is twice the head-on displayluminance measured in nits; and FIG. 8I is a graph illustrating asimulated lateral profile for zero degrees elevation of the visualsecurity factor of the arrangement of FIG. 8A and TABLE 1A for anambient illuminance measured in lux that is twice the head-on displayluminance measured in nits. Thus the arrangement of FIG. 8A and TABLE 1Ausing a typical luminance profile of an emissive display of FIG. 1A canachieve a privacy switch-on angle 140 of approximately 37 degrees.Advantageously an observer at 45 degrees polar angle for zero degreeselevation can see substantially no image information, independent ofimage content for such ambient lighting conditions. Further the displayachieves increased security level in the upper quadrants (azimuthalangles around +/−45 degrees from the northerly azimuthal direction). Inoperation, privacy displays desirably provide higher security levels forsnoopers in look-down viewing quadrants. In said look down quadrantshigh security factor is desirably achieved.

FIG. 8J is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement of FIG. 8A for an ambientilluminance measured in lux that is twice the head-on display luminancemeasured in nits and operated in public mode. Advantageously imagevisibility (S<0.1) is maintained to wide viewing angles so that thedisplay can be easily seen with high contrast from polar viewing anglesthat are greater than 45 degrees.

Other illustrate embodiments are illustrated in TABLES 1B-F and TABLE 2compares the privacy switch-on angle 140 for each illustrativeembodiment. In comparison to the arrangement of TABLE 1A, embodimentscomprising a C-plate in comparison to the crossed A-plates mayadvantageously be thinner and cheaper. Embodiments comprising twohomeotropic alignment layers in comparison to the two homogeneousalignment layers may advantageously achieve public mode operation withzero volts, and may have lower power consumption. Embodiments whereinthe first retarder 300A comprises a homeotropic alignment layer and ahomogeneous alignment layer advantageously achieve lower privacyswitch-on angle 140 while maintaining high security level at higherpolar angles.

TABLE 1B Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1250 nm 417BAHomeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330B Negative −1000nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm) 300A 417AB Homogeneous2 0 700 nm 417AA Homogeneous 2 180 {550 nm~800 nm} (450 nm~900 nm) 330AAPositive +570 nm A-plate {+400~625 nm} @ 45° (+250 nm~+800 nm) 330ABPositive +570 nm A-plate {+400~625 nm} @ 135° (+250 nm~+800 nm)

In respect of said other (with the same type of alignment layers) of theat least one first polar control retarder and the at least one secondpolar control retarder, both of the surface alignment layers arearranged to provide homogeneous alignment in the adjacent liquid crystalmaterial.

By way of comparison with the illustrative embodiment of TABLE 1A, TABLE1B provides the switchable liquid crystal retarder 314B of said one(with a homeotropic alignment layer and a homogeneous alignment layer)second polar control retarder has a retardance for light of a wavelengthof 550 nm having a first retardance value and the switchable liquidcrystal retarder of said other (with two homogeneous alignment layers)of the first polar control retarder 300B has a retardance for light of awavelength of 550 nm has a second retardance value, the first retardancevalue being greater than the second retardance value.

In respect of said other (with the same type of alignment layers) of theat least one first polar control retarder and the at least one secondpolar control retarder, being the second polar control retarder 300A inTABLE 1A and the first polar control retarder in TABLE 1B, both of thesurface alignment layers are arranged to provide homogenous alignment inthe adjacent liquid crystal material which has a retardance for light ofa wavelength of 550 nm in a range from 450 nm to 900 nm, preferably in arange from 550 nm to 800 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder further comprises a pair of passive uniaxial retarders havingoptical axes in the plane of the retarders that are crossed and eachhaving a retardance for light of a wavelength of 550 nm in a range from250 nm to 800 nm, preferably in a range from 400 nm to 625 nm.

TABLE 1C Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 0 750 nm 417BAHomogeneous 2 180 {600 nm~850 nm} (500 nm~900 nm) 330B Negative −440 nmC-plate {−350 nm~−600 nm} (−300 nm~−700 nm) 300A 417AB Homogeneous 2 1801250 nm 417AA Homeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330ANegative −1000 nm C-plate {−700 nm~−1700 nm} (~400 nm~−2100 nm)

TABLE 1D Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1250 nm 417BAHomeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330B Negative −1000nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm) 300A 417AB Homogeneous2 0 750 nm 417AA Homogeneous 2 180 {600 nm~850 nm} (500 nm~900 nm) 330ANegative −440 nm C-plate {−350 nm~−600 nm} (−300 nm~−700 nm)

For the illustrative embodiments of TABLES 1C-D in respect of the otherof the at least one first polar control retarder and the at least onesecond polar control retarder, both of the surface alignment layers arearranged to provide homogenous alignment in the adjacent liquid crystalmaterial which has a retardance for light of a wavelength of 550 nm in arange from 500 nm to 900 nm, preferably 600 nm to 850 nm, and said otherof the at least one first polar control retarder and the at least onesecond polar control retarder further comprises a passive uniaxialretarder having its optical axis perpendicular to the plane of theretarder and having a retardance for light of a wavelength of 550 nm ina range from −300 nm to −700 nm, preferably in a range from −350 nm to−600 nm.

TABLE 1E Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1250 nm 417BAHomeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330B Negative −1000nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm) 300A 417AB Homeotropic88 0 750 nm 417AA Homeotropic 88 180 {600 nm~850 nm} (500 nm~900 nm)330A Negative −660 nm C-plate {−400 nm~−800 nm} (−300 nm~−900 nm)

TABLE 1F Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homeotropic 88 0 750 nm 417BAHomeotropic 88 180 {600 nm~850 nm} (500 nm~900 nm) 330B Negative −660 nmC-plate {−400 nm~−800 nm} (−300 nm~−900 nm) 300A 417AB Homogeneous 2 1801250 nm 417AA Homeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330ANegative −1000 nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm)

For the illustrative embodiments of TABLES 1E-F, in respect of the otherof the at least one first polar control retarder and the at least onesecond polar control retarder, both of the surface alignment layers arearranged to provide homeotropic alignment in the adjacent liquid crystalmaterial which has a retardance for light of a wavelength of 550 nm in arange from 500 nm to 900 nm, preferably in a range from 600 nm to 850nm, and said other of the at least one first polar control retarder andthe at least one second polar control retarder further comprises apassive uniaxial retarder having its optical axis perpendicular to theplane of the retarder and having a retardance for light of a wavelengthof 550 nm in a range from −300 nm to −900 nm, preferably in a range from−400 nm to −800 nm.

TABLE 2 300A 300B Parallel cell alignment alignment half width layers &layers & 301A 330A 301B 330B angle 140 for passive passive retardance/retardance/ retardance/ retardance/ S = 1 @ TABLE retarder(s)retarder(s) nm nm nm nm 2 lux/nit/deg 1A Homogeneous Homogeneous 1250−1000 700 570 36.9 Homeotropic Homogeneous Negative C Crossed A 1BHomogeneous Homogeneous 700 570 1250 −1000 37.2 Homogeneous HomeotropicCrossed A Negative C 1C Homogeneous Homogeneous 1250 −1000 750 −440 37.8Homeotropic Homogeneous Negative C Negative C 1D Homogeneous Homogeneous750 −440 1250 −1000 37.9 Homogeneous Homeotropic Negative C Negative C1E Homeotropic Homogeneous 750 −660 1250 −1000 39.2 HomeotropicHomeotropic Negative C Negative C 1F Homogeneous Homeotropic 1250 −1000750 −660 39.6 Homeotropic Homeotropic Negative C Negative C

TABLE 2 illustrates that privacy switch-on 140 may be achieved forangles of less than 40 degrees. Advantageously a snooper at 45 degreespolar angle may have low image visibility at at least some azimuthalangles.

In alternative embodiments of the present disclosure illustrated in FIG.4A and FIG. 5A, the display device may further comprise a backlight 20arranged to output light, the spatial light modulator 48 comprises atransmissive spatial light modulator 48 arranged to receive output lightfrom the backlight 20, and said other (with alignment layers that are ofthe same type) of the at least one first polar control retarder and theat least one second polar control retarder is between the backlight 20and the transmissive spatial light modulator 48. Advantageously thevisibility of flow due to high restoration times of homeotropicalignment layers may be reduced.

It may be desirable to provide a display with a small switch-on angle140 while maintaining high security level for off-axis viewing. Theillustrative embodiment of TABLE 3A will now be discussed with referenceto FIGS. 9A-H.

In the embodiments of TABLES 3A-D, the switchable liquid crystalretarder of said one (with a homeotropic alignment layer and ahomogeneous alignment layer) of the at least one first polar controlretarder and the at least one second polar control retarder has aretardance for light of a wavelength of 550 nm having a first retardancevalue and the switchable liquid crystal retarder of said other (withalignment layers that are of the same type) the at least one first polarcontrol retarder and the at least one second polar control retarder hasa retardance for light of a wavelength of 550 nm has a second retardancevalue, half of the first retardance value being less than the secondretardance value.

TABLE 3A Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1480 nm 417BAHomogeneous 2 0 {850 nm~1800 nm} (700 nm~2500 nm) 330BA Positive +1050nm A-plate {+750 nm to +1300 nm} @ 45° (+600 nm to +1600 nm) 330BBPositive +1050 nm A-plate {+750 nm to +1300 nm} @ 135° (+600 nm to +1600 nm) 300A 417AB Homogeneous 2 180 1250 nm 417AA Homeotropic 88 0{850 nm~1800 nm} (700 nm~2500 nm) 330A Negative −1000 nm C-plate {−700nm~−1700 nm} (−400 nm~−2100 nm)

FIG. 9A is a graph illustrating a simulated polar profile of luminanceoutput of a transmissive spatial light modulator 48 that is illuminatedby a collimated backlight 20. The display device 100 thus furthercomprises a backlight 20 arranged to output light, and the spatial lightmodulator 48 comprises a transmissive spatial light modulator arrangedto receive output light from the backlight. The backlight has an outputluminance profile having a full width half maximum that is at most 40degrees. The spatial light modulator may alternatively comprise anemissive spatial light modulator and a parallax barrier 70) as describedwith respect to FIGS. 12A-B, below.

The optical stack of FIG. 8A comprising the layers of TABLE 3A and acollimated backlight 20 will now be discussed.

FIG. 9B is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder of TABLE 3A; FIG. 9Cis a graph illustrating a simulated polar profile of reflectivity of thesecond polar control retarder of TABLE 3A; FIG. 9D is a graphillustrating a simulated polar profile of the total reflectivitycomprising the reflectivity of FIG. 9C and the Fresnel reflectivity fromthe front surface of the display device.

FIG. 9E is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder of TABLE 3A. Thepretilt directions of the first polar control retarder 300A are parallelor anti-parallel to the pretilt directions of the second polar controlretarder 300B.

FIG. 9F is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders for the polar profiles of FIGS.9A-E.

FIG. 9G is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement of FIG. 8A, and the profiles ofFIGS. 9A-F for an ambient illuminance measured in lux that is twice thehead-on display luminance measured in nits for the polar profiles ofFIGS. 9A-E; and FIG. 9H is a graph illustrating a simulated lateralprofile for zero degrees elevation of the visual security factor of thearrangement for an ambient illuminance measured in lux that is twice thehead-on display luminance measured in nits for the polar profiles ofFIGS. 9A-E.

Other illustrate embodiments are illustrated in TABLES 3B-D and TABLE 4compares the privacy switch-on angle 140 for each illustrativeembodiment. The embodiments may be provided for the arrangements ofFIGS. 1A-B or alternatively for the arrangements of FIGS. 4A-B, whereinthe arrangements comprise reflective polariser 302.

In comparison to the arrangement of TABLE 3A, embodiments comprising aC-plate in comparison to the crossed A-plates may advantageously bethinner and cheaper. Embodiments comprising two homeotropic alignmentlayers in comparison to the two homogeneous alignment layers mayadvantageously achieve public mode operation with zero volts, and mayhave lower power consumption. Embodiments wherein the first retarder300A comprises a homeotropic alignment layer and a homogeneous alignmentlayer advantageously achieve lower privacy switch-on angle 140 whilemaintaining high security level at higher polar angles.

TABLE 3B Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homeotropic 88 0 1500 nm 417BAHomeotropic 88 180 {1000 nm~1800 nm} (700 nm~2500 nm) 330B Negative−1400 nm C-plate {−900 nm~−1800 nm} (−700 nm~−2500 nm) 300A 417ABHomogeneous 2 180 1250 nm 417AA Homeotropic 88 0 {850 nm~1800 nm} (700nm~2500 nm) 330A Negative −1000 nm C-plate {−700 nm~−1700 nm} (−400nm~−2100 nm)

TABLE 3C Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1250 nm 417BAHomeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330B Negative −1000nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm) 300A 417AB Homogeneous2 180 1480 nm 417AA Homogeneous 2 0 {850 nm~1800 nm} (700 nm~2500 nm)330AB Positive +1050 nm A-plate {+750 nm to +1300 nm} @ 45° (+600 nm to+1600 nm) 330AA Positive +1050 nm A-plate {+750 nm to +1300 nm} @ 135°(+600 nm to +1600 nm)

TABLE 3D Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1250 nm 417BAHomeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330B Negative −1000nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm) 300A 417AB Homeotropic88 0 1500 nm 417AA Homeotropic 88 180 {1000 nm~1800 nm} (700 nm~2500 nm)330A Negative −1400 nm C-plate {−900 nm~−1800 nm} (−700 nm~−2500 nm)

For the illustrative embodiments of TABLES 3A-D, said one retarder (witha homeotropic alignment layer and a homogeneous alignment layer) has aliquid crystal retarder 314 that has a retardance for light of awavelength of 550 nm having a first retardance value in a range from 700nm to 2500 nm, preferably in a range from 850 nm to 1800 nm.

In respect of the other of the at least one first polar control retarderand the at least one second polar control retarder, wherein both of thesurface alignment layers are arranged to provide homogenous alignment inthe adjacent liquid crystal material which has a retardance for light ofa wavelength of 550 nm in a range from 700 nm to 2500 nm, preferably ina range from 850 nm to 1800 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder further comprises a pair of passive uniaxial retarders havingoptical axes in the plane of the retarders that are crossed and eachhaving a retardance for light of a wavelength of 550 nm in a range from600 nm to 1600 nm, preferably in a range from 750 nm to 1300 nm.

In respect of the other of the at least one first polar control retarderand the at least one second polar control retarder, wherein both of thesurface alignment layers are arranged to provide homeotropic alignmentin the adjacent liquid crystal material which has a retardance for lightof a wavelength of 550 nm in a range from 700 nm to 2500 nm, preferablyin a range from 1000 nm to 1800 nm, and said other of the at least onefirst polar control retarder and the at least one second polar controlretarder further comprises a passive uniaxial retarder having itsoptical axis perpendicular to the plane of the retarder and having aretardance for light of a wavelength of 550 nm in a range from −700 nmto −2500 nm, preferably in a range from −900 nm to −1800 nm.

TABLE 4 300A 300B Parallel cell alignment alignment half width layers &layers & 301A 330A 301B 330B angle 140 for passive passive retardance/retardance/ retardance/ retardance/ S = 1 @ TABLE retarder(s)retarder(s) nm nm nm nm 2 lux/nit/deg 3A Homogeneous Homogeneous 1250−1000 1480 1050 24.9 Homeotropic Homogeneous Negative C Crossed A 3BHomogeneous Homeotropic 1250 −1000 1500 −1400 26.4 HomeotropicHomeotropic Negative C Negative C 3C Homogeneous Homogeneous 1480 10501250 −1000 26.8 Homogeneous Homeotropic Crossed A Negative C 3DHomeotropic Homogeneous 1500 −1400 1250 −1000 27.7 HomeotropicHomeotropic Negative C Negative C

FIG. 9I is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement of FIG. 9A for an ambientilluminance measured in lux that is twice the head-on display luminancemeasured in nits and operated in public mode. In comparison to thearrangement using the wide angle emission of FIG. 8B, the collimatedbacklight with luminance profile of FIG. 9A achieves improved privacyviewing angles, but has narrower polar range over which other observerscan see a high contrast image (S<0.1) in privacy mode. Such a display iswell suited to display applications that are primarily for singleviewers with occasional use by multiple users.

It may be desirable to provide a switchable privacy display with noreflective polariser 302. The illustrative embodiment of TABLE 3A willnow be discussed with reference to FIGS. 10A-H. Further The arrangementsof TABLES 3B-D may be provided in such embodiments.

FIG. 10A is a graph illustrating a simulated polar profile of luminanceoutput of a transmissive spatial light modulator that is illuminated bya collimated backlight: The display may comprise the arrangement ofFIGS. 5A-B. Alternatively the arrangement of FIGS. 1A-B provided with noreflective polariser 302 and with the spatial light modulator of FIG. 3.Alternatively the arrangement of FIGS. 4A-B may be provided with noreflective polariser 302.

FIG. 10B is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder: FIG. 10C is a graphillustrating a simulated polar profile of reflectivity of the secondpolar control retarder for no reflective polariser; FIG. 10D is a graphillustrating a simulated polar profile of the total reflectivitycomprising the reflectivity of FIG. 9C and the Fresnel reflectivity fromthe front surface of the display device for no reflective polariser.

FIG. 10E is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder. The pretilt directionsof the first polar control retarder 300A are parallel or anti-parallelto the pretilt directions of the second polar control retarder 300B.

FIG. 10F is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders for the polar profiles of FIGS.10A-E.

FIG. 10G is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement with no reflective polariser for anambient illuminance measured in lux that is twice the head-on displayluminance measured in nits; and FIG. 10H is a graph illustrating asimulated lateral profile for zero degrees elevation of the visualsecurity factor of the arrangement for an ambient illuminance measuredin lux that is twice the head-on display luminance measured in nits.

The relative performance of switch-on angle 140 is illustrated in TABLE5. The order of the first and second polar control retarders 300A, 300Bdoes not change the output polar profiles.

TABLE 5 First Second First Second Parallel cell alignment alignmentpassive passive half width layers & layers & First LC retarder(s) SecondLC retarder(s) angle 140 for passive passive retardance/ retardance/retardance/ retardance/ S = 1 @ TABLE retarder(s) retarder(s) nm nm nmnm 2 lux/nit/deg 3A, 3C Homogeneous Homogeneous 1250 −1000 1480 105029.5 Homeotropic Homogeneous Negative C Crossed A 3B, 3D Homeotropic1500 −1400 30.7 Homeotropic Negative C

Advantageously a display may be provided with lower reflectivity incomparison to arrangements with reflective polariser 302. Sucharrangements may be desirable in certain operating environments such asautomotive cabins in bright sunlight.

It may be desirable to provide privacy displays that achieve highsecurity level for off-axis viewing in both landscape and portrait modesof operation. The illustrative embodiment of TABLE 6A will now bediscussed with reference to FIGS. 11A-G.

TABLE 6A Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 0 700 nm 417BAHomogeneous 2 180 {550 nm~800 nm} (450 nm~900 nm) 330BA Positive +570 nmA-plate {+400~625 nm} @ 45° (+250 nm~+800 nm) 330BB Positive +570 nmA-plate {+400~625 nm} @ 135° (+250 nm~+800 nm) 300A 417AB Homogeneous 290 1250 nm 417AA Homeotropic 88 270 {850 nm~1800 nm} (700 nm~2500 nm)330A Negative −1000 nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm)

In respect of the other (with alignment layers that are of the sametype) of the at least one first polar control retarder and the at leastone second polar control retarder, both of the surface alignment layersare arranged to provide homogenous alignment in the adjacent liquidcrystal material which has a retardance for light of a wavelength of 550nm in a range from 450 nm to 900 nm, preferably in a range from 550 nmto 800 nm, and said other of the at least one first polar controlretarder and the at least one second polar control retarder furthercomprises a pair of passive uniaxial retarders having optical axes inthe plane of the retarders that are crossed and each having a retardancefor light of a wavelength of 550 nm in a range from 250 nm to 800 nm,preferably in a range from 400 nm to 625 nm.

FIG. 11A is a graph illustrating a simulated polar profile of luminanceoutput of a transmissive spatial light modulator that is illuminated bya collimated backlight; FIG. 11B is a graph illustrating a simulatedpolar profile of transmission of the second polar control retarder: FIG.11C is a graph illustrating a simulated polar profile of reflectivity ofthe second polar control retarder: and FIG. 11D is a graph illustratinga simulated polar profile of the total reflectivity comprising thereflectivity of FIG. 11C and the Fresnel reflectivity from the frontsurface of the display device.

Said surface alignment layers of said other (with alignment layers thatare of the same type) of the at least one first polar control retarderand the at least one second polar control retarder have pretilts havingpretilt directions with components in the plane of the layer of liquidcrystal material in a second pair of anti-parallel directions.

FIG. 11E is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder wherein the pretiltdirections of the first polar control retarder are orthogonal to thepretilt directions of the second polar control retarder. The pretiltdirections of the first polar control retarder 300A are orthogonal tothe pretilt directions of the second polar control retarder 300B. Saidsurface alignment layers of said one (with a homeotropic alignment layerand a homogeneous alignment layer) of the at least one first polarcontrol retarder and the at least one second polar control retarder havepretilts having pretilt directions with components in the plane of thelayer of liquid crystal material in a first pair of anti-paralleldirections.

In the illustrative embodiments of FIGS. 11A-G and FIGS. 12A-J, thefirst pair of anti-parallel directions are crossed with the second pairof anti-parallel directions. The first pair of anti-parallel directionsare at 90 degrees to the second pair of anti-parallel directions, asviewed normal the planes of the layers of liquid crystal material of theat least one first polar control retarder and the at least one secondpolar control retarder.

The switchable liquid crystal retarder of said one (with a homeotropicalignment layer and a homogeneous alignment layer) of the at least onefirst polar control retarder and the at least one second polar controlretarder has a retardance for light of a wavelength of 550 nm having afirst retardance value and the switchable liquid crystal retarder ofsaid other (with alignment layers that are of the same type) the atleast one first polar control retarder and the at least one second polarcontrol retarder has a retardance for light of a wavelength of 550 nmhas a second retardance value, the first retardance value being greaterthan the second retardance value.

The magnitude of the difference between half the first retardance valueand the second retardance value is at most 400 nm and preferably at most200 nm.

FIG. 11F is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders.

FIG. 11G is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement with no reflective polariser for anambient illuminance measured in lux that is twice the head-on displayluminance measured in nits for the polar profiles of FIGS. 11A-F.

Advantageously a display may be provided with high visual security forlandscape and portrait operation of a display in privacy mode, with highsecurity factor for off-axis viewing locations. Such a display may beswitched to a public mode of operation with security level similar tothat illustrated in FIG. 9J.

Other illustrate embodiments are illustrated in TABLES 6B-C. Theembodiments may be provided for the arrangements of FIGS. 1A-B oralternatively for the arrangements of FIGS. 4A-B, wherein thearrangements comprise reflective polariser 302. In other embodiments,reflective polariser 302 may be omitted.

In comparison to the arrangement of TABLE 6A, embodiments comprising aC-plate in comparison to the crossed A-plates may advantageously bethinner and cheaper. Embodiments comprising two homeotropic alignmentlayers in comparison to the two homogeneous alignment layers mayadvantageously achieve public mode operation with zero volts, and mayhave lower power consumption. Embodiments wherein the first retarder300A comprises a homeotropic alignment layer and a homogeneous alignmentlayer advantageously achieve lower privacy switch-on angle 140 whilemaintaining high security level at higher polar angles.

TABLE 6B Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 0 750 nm 417BAHomogeneous 2 180 {600 nm~850 nm} (500 nm~900 nm) 330B Negative −440 nmC-plate {−350 nm~−600 nm} (−300 nm~−700 nm) 300A 417AB Homogeneous 2 901250 nm 417AA Homeotropic 88 270 {850 nm~1800 nm} (700 nm~2500 nm) 330ANegative −1000 nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm)

In respect of the other (with alignment layers that are of the sametype) of the at least one first polar control retarder and the at leastone second polar control retarder, both of the surface alignment layersare arranged to provide homogenous alignment in the adjacent liquidcrystal material which has a retardance for light of a wavelength of 550nm in a range from 500 nm to 900 nm, preferably in a range from 600 nmto 850 nm, and said other of the at least one first polar controlretarder and the at least one second polar control retarder furthercomprises a passive uniaxial retarder having its optical axisperpendicular to the plane of the retarder and having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −700 nm,preferably in a range from −350 nm to −600 nm.

TABLE 6C Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1250 nm 417BAHomeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330B Negative −1000nm C-plate {−700 nm~−1700 nm} (−400 nm~ −2100 nm) 300A 417AB Homeotropic88 90 750 nm 417AA Homeotropic 88 270 {600 nm~850 nm} (500 nm~900 nm)330A Negative −660 nm C-plate {−400 nm~−800 nm} (−300 nm~−900 nm)

In respect of the other (with alignment layers that are of the sametype) of the at least one first polar control retarder and the at leastone second polar control retarder, both of the surface alignment layersare arranged to provide homeotropic alignment in the adjacent liquidcrystal material which has a retardance for light of a wavelength of 550nm in a range from 500 nm to 900 nm, preferably in a range from 600 nmto 850 nm, and said other of the at least one first polar controlretarder and the at least one second polar control retarder furthercomprises a passive uniaxial retarder having its optical axisperpendicular to the plane of the retarder and having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −800 nm,preferably in a range from −400 nm to −800 nm.

TABLE 7 compares the retardances of the embodiments of FIGS. 6A-C.

TABLE 7 First Second First Second alignment alignment passive passivelayers & layers & First LC retarder(s) Second LC retarder(s) passivepassive retardance/ retardance/ retardance/ retardance/ TABLEretarder(s) retarder(s) nm nm nm nm 6A Homogeneous Homogeneous 1250−1000 700 570 Homeotropic Homogeneous Negative C Crossed A 6BHomogeneous 750 −440 Homogeneous Negative C 6C Homeotropic 1500 −1400Homeotropic Negative C

It may be desirable to provide privacy displays that achieve highsecurity level for off-axis viewing in both landscape and portrait modesof operation for an emissive display. The illustrative embodiment ofTABLE 9A will now be discussed with reference to FIGS. 12A-G for anillustrative pixel plane 214 and parallax barrier 700 arrangement ofFIG. 2.

FIG. 12A is a graph illustrating a simulated polar profile of luminanceoutput of an emissive spatial light modulator.

The spatial light modulator 48 comprises an emissive spatial lightmodulator arranged to output light, the display polariser is an outputdisplay polariser 218 arranged on the output side of the emissivespatial light modulator 48, as illustrated in FIG. 1A. The emissivespatial light modulator comprises an array of pixels 220, 222, 224arranged in a pixel layer 214, and the display device further comprisesa parallax barrier 700 forming an array of apertures 702, wherein theparallax barrier 700 is separated from the pixel layer 214 by a parallaxdistance d along an axis along a normal to the plane of the pixel layer214, each pixel 220, 222, 224 being aligned with an aperture 702.

FIG. 12B is a graph illustrating a simulated polar profile oftransmission of the barrier structure of FIG. 2 of light from the pixelsof the emissive spatial light modulator. An illustrative example isprovided in TABLE 8 where the emissive spatial light modulator 48 andthe aligned parallax barrier 700 has an output luminance profile havinga full width half maximum that is at most 40 degrees.

TABLE 8 Parameter, x-axis direction Illustrative value Pixel 224 pitch20 microns Pixel 224 emitting width 10 microns Barrier aperture 702width 10 microns Barrier separation, d 20 microns

TABLE 9A Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 0 480 nm 417BAHomogeneous 2 180 {400 nm~600 nm} (300 nm~800 nm) 330BA Positive +570 nmA-plate {+250~625 nm} @ 45° (+150 nm~+800 nm) 330BB Positive +570 nmA-plate {+250~625 nm} @ 135° (+150 nm~+800 nm) 300A 417AB Homogeneous 2180 1250 nm 417AA Homeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm)330A Negative −1000 nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm)

FIG. 12C is a graph illustrating a simulated polar profile oftransmission of the second polar control retarder arranged between thefirst and second additional polarisers wherein the electric vectortransmission directions of the polarisers are parallel, FIG. 12D is agraph illustrating a simulated polar profile of reflectivity of thesecond polar control retarder arranged between a reflective polariserand the second additional polariser wherein the electric vectortransmission directions of the polarisers are parallel; and FIG. 12E isa graph illustrating a simulated polar profile of the total reflectivitycomprising the reflectivity and the Fresnel reflectivity from the frontsurface of the display device.

FIG. 12F is a graph illustrating a simulated polar profile oftransmission of the first polar control retarder arranged between thedisplay polariser and the first additional polariser wherein theelectric vector transmission directions of the polarisers are paralleland the pretilt directions of the first polar control retarder 300A areorthogonal to the pretilt directions of the second polar controlretarder 300B.

FIG. 12G is a graph illustrating a simulated polar profile of thelogarithm of total output luminance of the spatial light modulator andfirst and second polar control retarders as illustrated in FIGS. 12A-F;and FIG. 12H is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement for the polar profiles of FIGS.12A-G for an ambient illuminance measured in lux that is twice thehead-on display luminance measured in nits.

Advantageously a display that provides desirable security factor, S inlandscape and portrait operation may be provided. In comparison to theembodiment of TABLE 6A, the desirable retardance of the first polarcontrol retarder is reduced. Such a polar control retarder provides highangle luminance reduction, while the parallax barrier 700 provides lowerangle luminance reduction. Advantageously increased uniformity ofsecurity level is achieved in an emissive display.

FIG. 12I is a graph illustrating a simulated polar profile of thesecurity level, S wherein the first polar control retarder is the secondpolar control retarder of FIG. 11H and the second polar control retarderis the first polar control retarder of FIG. 11H, as illustrated in TABLE9B. In comparison to the arrangement of TABLE 9A, advantageously privacyswitch-on angle is reduced in the lateral direction.

TABLE 9B Additional passive LC layer 314 Additional retarder 330In-plane retardance passive retardance Alignment alignment {narrowrange} retarder {narrow range} Item Layer type Pretilt direction (widerange) 330 type (wide range) 300B 417BB Homogeneous 2 180 1250 nm 417BAHomeotropic 88 0 {850 nm~1800 nm} (700 nm~2500 nm) 330BA Negative −1000nm C-plate {−700 nm~−1700 nm} (−400 nm~−2100 nm) 300A 417AB Homogeneous2 0 480 nm 417AA Homogeneous 2 180 {400 nm~600 nm} (300 nm~800 nm) 330AAPositive +570 nm A-plate {+250~625 nm} @ 45° (+150 nm~+800 nm) 330ABPositive +570 nm A-plate {+250~625 nm} @ 135° (+150 nm~+800 nm)

In other embodiments the first polar control retarder may comprisehomeotropic alignment layers, to advantageously achieve reduced powerconsumption. In other embodiments the first polar control retarder maycomprise passive C-plate retarders to advantageously achieve reducedcost and complexity as described elsewhere herein.

FIG. 12J is a graph illustrating a simulated polar profile of thesecurity level, S of the arrangement for an ambient illuminance measuredin lux that is twice the head-on display luminance measured in nits andoperated in public mode for the arrangements of FIGS. 12A-H.Advantageously image visibility (S<0.1) is maintained to so that thedisplay can be easily seen with high contrast from polar viewing anglesthat are greater than 30 degrees in both landscape and portraitorientations.

FIG. 13 is a schematic diagram illustrating in side perspective view aswitchable privacy display component 102 for use with a spatial lightmodulator 48 comprising a first polar control retarder 300A and a firstadditional polariser 318A, a reflective polariser 302; and a secondpolar control retarder 300B arranged between the first additionalpolariser 318A and a second additional polariser 318B. Advantageously aspatial light modulator 48 may be reconfigured in the factory or in thefield for use as a switchable privacy display. Features of theembodiment of FIG. 13 not discussed in further detail may be assumed tocorrespond to the features with equivalent reference numerals asdiscussed above, including any potential variations in the features.

Various alternatives for optical stackings of the present embodimentswill now be described

FIG. 14 is a key for the alternative stacking arrangements of FIGS.15A-F, FIGS. 16A-F, FIGS. 17A-C and FIGS. 18A-F.

FIGS. 15A-F are schematic diagrams illustrating in side viewalternatives for optical component stackings for a switchable privacydisplay 100 wherein the first and second polar control retarders 300A,300B are arranged to receive light from a transmissive spatial lightmodulator 48 and backlight 20. Features of the embodiment of FIGS. 15A-Fnot discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

FIGS. 16A-F are schematic diagrams illustrating in side viewalternatives for optical component stackings for a switchable privacydisplay 100 wherein one of the first and second polar control retarders300A, 300B is arranged to receive light from the transmissive spatiallight modulator 48 and the spatial light modulator 48 is arranged toreceive light from the other of the first and second polar controlretarders 300A, 300B and backlight 20, and FIGS. 17A-C are schematicdiagrams illustrating in side view alternatives for optical componentstackings for a switchable privacy display wherein the spatial lightmodulator 48 is arranged to receive light from the first and secondpolar control retarders 300A, 300B and backlight 20. Features of theembodiment of FIGS. 16A-F and FIGS. 17A-C not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

FIGS. 18A-F are schematic diagrams illustrating in side viewalternatives for optical component stackings for a switchable privacydisplay 100 wherein the first and second polar control retarders 300A,300B are arranged to receive light from an emissive spatial lightmodulator 48. Features of the embodiment of FIGS. 18A-F not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

In the alternatives of FIGS. 15A-F, FIGS. 16A-F, FIGS. 17A-C and FIGS.18A-F, various alternatives are illustrated and can more generally bedescribed as follows.

For each polar control retarder 300A, 300B that comprises a liquidcrystal retarder 314A, 314B with two homogeneous alignment layers 417A,417B then the respective passive retarder 330A, 330B or pair of crossedpassive retarders 330AA, 330AB or 330BA, 330BB may be arranged to eitherreceive light from the respective liquid crystal retarder 314A, 314B; orthe liquid crystal retarder 314A, 314B may be arranged to receive lightfrom the respective passive retarder 330A, 330B or pair of crossedpassive retarders 330AA, 330AB or 330BA, 330BB.

For each polar control retarder 300A, 300B that comprises a liquidcrystal retarder 314A, 314B with one homogeneous alignment layer and onehomeotropic alignment layer 417A, 417B then preferably the homeotropicalignment layer is arranged between the respective layer of liquidcrystal material 421A, 421B and the respective passive retarder 330A,330B. Advantageously the size of the polar region for reduced luminancein privacy mode is increased.

For the alternatives of FIGS. 15A, 15C, 15E, FIGS. 16A, 16C. 16E, andFIGS. 18A, 18C, 18E, one of polar control retarders 300A, 300B may bearranged between reflective polariser 302 and further additionalpolariser 318B. Advantageously the reflectivity of the display 100 inprivacy mode of operation may be increased, and security factorincreased.

The alternatives of FIGS. 15A-F, FIGS. 16A-F and FIGS. 17A-C with thetransmissive spatial light modulator 48 and backlight 20 omitted or thealternative of FIGS. 18A-F with the emissive spatial light modulator 48omitted may provide alternative components such as that illustrated inFIG. 13.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1. A display device comprising: a spatial light modulator; a displaypolariser arranged on a side of the spatial light modulator, the displaypolariser being a linear polariser; a first additional polariserarranged on the same side of the spatial light modulator as the displaypolariser, the first additional polariser being a linear polariser; atleast one first polar control retarder arranged between the firstadditional polariser and the display polariser, a second additionalpolariser, the second additional polariser being a linear polariser; andat least one second polar control retarder, wherein either: the secondadditional polariser is arranged on the same side of the spatial lightmodulator as the first additional polariser outside the first additionalpolariser, and the at least one second polar control retarder isarranged between the first additional polariser and the secondadditional polariser; or the display device further comprises abacklight arranged to output light, the spatial light modulatorcomprises a transmissive spatial light modulator arranged to receiveoutput light from the backlight, said display polariser is an inputdisplay polariser arranged on the input side of the spatial lightmodulator, and the display device further comprises an output displaypolariser arranged on the output side of the spatial light modulator,the second additional polariser is arranged on the output side of thespatial light modulator, and the at least one second polar controlretarder is arranged between the second additional polariser and theoutput display polariser, wherein: each of the at least one first polarcontrol retarder and the at least one second polar control retardercomprises a respective switchable liquid crystal retarder comprising alayer of liquid crystal material and two surface alignment layersdisposed adjacent to the layer of liquid crystal material and onopposite sides thereof, in respect of one of the at least one firstpolar control retarder and the at least one second polar controlretarder, one of the surface alignment layers is arranged to providehomogenous alignment in the adjacent liquid crystal material and theother of the surface alignment layers is arranged to provide homeotropicalignment in the adjacent liquid crystal material, and in respect of theother of the at least one first polar control retarder and the at leastone second polar control retarder, both of the surface alignment layersare arranged to provide homogenous alignment in the adjacent liquidcrystal material or both of the surface alignment layers are arranged toprovide homeotropic alignment in the adjacent liquid crystal material.2. A display device according to claim 1, wherein the switchable liquidcrystal retarder of said one of the at least one first polar controlretarder and the at least one second polar control retarder has aretardance for light of a wavelength of 550 nm having a first retardancevalue in a range from 700 nm to 2500 nm, preferably in a range from 850nm to 1800 nm.
 3. A display device according to claim 1, wherein saidone of the at least one first polar control retarder and the at leastone second polar control retarder further comprises at least one passivecompensation retarder.
 4. A display device according to claim 3, whereinthe at least one passive compensation retarder of said one of the atleast one first polar control retarder and the at least one second polarcontrol retarder is arranged on the same side of the switchable liquidcrystal retarder as the surface alignment layers that is arranged toprovide homeotropic alignment in the adjacent liquid crystal material.5. A display device according to claim 3, wherein the at least onepassive compensation retarder of said one of the at least one firstpolar control retarder and the at least one second polar controlretarder comprises a passive uniaxial retarder having its optical axisperpendicular to the plane of the retarder.
 6. A display deviceaccording to claim 5, wherein the passive uniaxial retarder has aretardance for light of a wavelength of 550 nm in a range from −400 nmto −2100 nm, preferably in a range from −700 nm to −1700 nm.
 7. Adisplay device according to claim 1, wherein the display device furthercomprises a reflective polariser, the reflective polariser being alinear polariser, and either: said display polariser is an outputdisplay polariser arranged on the output side of the spatial lightmodulator, the second additional polariser is arranged on the same sideof the spatial light modulator as the first additional polariser outsidethe first additional polariser, the at least one second polar controlretarder is arranged between the first additional polariser and thesecond additional polariser, and the reflective polariser is arrangedbetween the first additional polariser and the at least one second polarcontrol retarder; or the display device further comprises a backlightarranged to output light, the spatial light modulator comprises atransmissive spatial light modulator arranged to receive output lightfrom the backlight, said display polariser is an input display polariserarranged on the input side of the spatial light modulator, and thedisplay device further comprises an output display polariser arranged onthe output side of the spatial light modulator, the second additionalpolariser is arranged on the output side of the spatial light modulator,the at least one second polar control retarder is arranged between thesecond additional polariser and the output display polariser, and thereflective polariser is arranged between the output display polariserand the at least one second polar control retarder.
 8. A display deviceaccording to claim 7, wherein said one of the at least one first polarcontrol retarder and the at least one second polar control retarder isthe at least one second polar control retarder and said other of the atleast one first polar control retarder and the at least one second polarcontrol retarder is the at least one first polar control retarder.
 9. Adisplay device according to claim 1, wherein the switchable liquidcrystal retarder of said one of the at least one first polar controlretarder and the at least one second polar control retarder has aretardance for light of a wavelength of 550 nm having a first retardancevalue and the switchable liquid crystal retarder of said other pf the atleast one first polar control retarder and the at least one second polarcontrol retarder has a retardance for light of a wavelength of 550 nmhaving a second retardance value, the first retardance value beinggreater than the second retardance value.
 10. A display device accordingto claim 9, wherein the magnitude of the difference between half thefirst retardance value and the second retardance value is at most 400nm.
 11. A display device according to claim 9, wherein in respect ofsaid other of the at least one first polar control retarder and the atleast one second polar control retarder, both of the surface alignmentlayers are arranged to provide homogenous alignment in the adjacentliquid crystal material which has a retardance for light of a wavelengthof 550 nm in a range from 450 nm to 900 nm, preferably in a range from550 nm to 800 nm, and said other of the at least one first polar controlretarder and the at least one second polar control retarder furthercomprises a pair of passive uniaxial retarders having optical axes inthe plane of the retarders that are crossed and each having a retardancefor light of a wavelength of 550 nm in a range from 250 nm to 800 nm,preferably in a range from 400 nm to 625 nm.
 12. A display deviceaccording to claim 9, wherein in respect of the other of the at leastone first polar control retarder and the at least one second polarcontrol retarder, both of the surface alignment layers are arranged toprovide homogenous alignment in the adjacent liquid crystal materialwhich has a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 900 nm, preferably in a range from 600 nm to 850 nm, andsaid other of the at least one first polar control retarder and the atleast one second polar control retarder further comprises a passiveuniaxial retarder having its optical axis perpendicular to the plane ofthe retarder and having a retardance for light of a wavelength of 550 nmin a range from −300 nm to −700 nm, preferably in a range from −350 nmto −600 nm.
 13. A display device according to claim 9, wherein inrespect of the other of the at least one first polar control retarderand the at least one second polar control retarder, both of the surfacealignment layers are arranged to provide homeotropic alignment in theadjacent liquid crystal material which has a retardance for light of awavelength of 550 nm in a range from 500 nm to 900 nm, preferably in arange from 600 nm to 850 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder further comprises a passive uniaxial retarder having itsoptical axis perpendicular to the plane of the retarder and having aretardance for light of a wavelength of 550 nm in a range from −300 nmto −900 nm, preferably in a range from −400 nm to −800 nm.
 14. A displaydevice according to claim 9, wherein the spatial light modulatorcomprises an emissive spatial light modulator arranged to output light,the display polariser is an output display polariser arranged on theoutput side of the emissive spatial light modulator, the secondadditional polariser is arranged on the output side of the spatial lightmodulator outside the first additional polariser, and the at least onesecond polar control retarder is arranged between the first additionalpolariser and the second additional polariser.
 15. A display deviceaccording to claim 14, wherein the emissive spatial light modulator hasan output luminance profile having a full width half maximum that is atleast 40 degrees, preferably at least 50 degrees.
 16. A display deviceaccording to claim 9, wherein the display device further comprises abacklight arranged to output light, and the spatial light modulatorcomprises a transmissive spatial light modulator arranged to receiveoutput light from the backlight.
 17. A display device according to claim16, wherein the backlight has an output luminance profile having a fullwidth half maximum that is at least 40 degrees, preferably at least 50degrees.
 18. A display device according to claim 1, wherein theswitchable liquid crystal retarder of said one of the at least one firstpolar control retarder and the at least one second polar controlretarder has a retardance for light of a wavelength of 550 nm having afirst retardance value and the switchable liquid crystal retarder ofsaid other the at least one first polar control retarder and the atleast one second polar control retarder has a retardance for light of awavelength of 550 nm has having a second retardance value, half of thefirst retardance value being less than the second retardance value. 19.A display device according to claim 18, wherein in respect of the otherof the at least one first polar control retarder and the at least onesecond polar control retarder, both of the surface alignment layers arearranged to provide homogenous alignment in the adjacent liquid crystalmaterial which has a retardance for light of a wavelength of 550 nm in arange from 700 nm to 2500 nm, preferably in a range from 850 nm to 1800nm, and said other of the at least one first polar control retarder andthe at least one second polar control retarder further comprises a pairof passive uniaxial retarders having optical axes in the plane of theretarders that are crossed and each having a retardance for light of awavelength of 550 nm in a range from 600 nm to 1600 nm, preferably in arange from 750 nm to 1300 nm.
 20. A display device according to claim18, wherein in respect of the other of the at least one first polarcontrol retarder and the at least one second polar control retarder,both of the surface alignment layers are arranged to provide homeotropicalignment in the adjacent liquid crystal material which has a retardancefor light of a wavelength of 550 nm in a range from 700 nm to 2500 nm,preferably in a range from 1000 nm to 1800 nm, and said other of the atleast one first polar control retarder and the at least one second polarcontrol retarder further comprises a passive uniaxial retarder havingits optical axis perpendicular to the plane of the retarder and having aretardance for light of a wavelength of 550 nm in a range from −700 nmto −2500 nm, preferably in a range from −900 nm to −1800 nm.
 21. Adisplay device according to claim 18, wherein the spatial lightmodulator comprises an emissive spatial light modulator arranged tooutput light, the display polariser is an output display polariserarranged on the output side of the emissive spatial light modulator, thesecond additional polariser is arranged on the output side of thespatial light modulator outside the first additional polariser, and theat least one second polar control retarder is arranged between the firstadditional polariser and the second additional polariser.
 22. A displaydevice according to claim 21, wherein the emissive spatial lightmodulator comprises an array of pixels arranged in a pixel layer, andthe display device further comprises a parallax barrier forming an arrayof apertures, wherein the parallax barrier is separated from the pixellayer by a parallax distance along an axis along a normal to the planeof the pixel layer, each pixel being aligned with an aperture.
 23. Adisplay device according to claim 22, wherein the emissive spatial lightmodulator and the parallax barrier has an output luminance profilehaving a full width half maximum that is at most 40 degrees.
 24. Adisplay device according to claim 18, wherein the display devicecomprises a backlight arranged to output light, and the spatial lightmodulator comprises a transmissive spatial light modulator arranged toreceive output light from the backlight.
 25. A display device accordingto claim 24, wherein the backlight has an output luminance profilehaving a full width half maximum that is at most 40 degrees.
 26. Adisplay device according to claim 1, wherein said surface alignmentlayers of said one of the at least one first polar control retarder andthe at least one second polar control retarder have pretilts havingpretilt directions with components in the plane of the layer of liquidcrystal material in a first pair of anti-parallel directions, and saidsurface alignment layers of said other of the at least one first polarcontrol retarder and the at least one second polar control retarder havepretilts having pretilt directions with components in the plane of thelayer of liquid crystal material in a second pair of anti-paralleldirections, the first pair of anti-parallel directions being crossedwith the second pair of anti-parallel directions.
 27. A display deviceaccording to claim 26, wherein the first pair of anti-paralleldirections are at 90 degrees to the second pair of anti-paralleldirections, as viewed normal the planes of the layers of liquid crystalmaterial of the at least one first polar control retarder and the atleast one second polar control retarder.
 28. A display device accordingto claim 26, wherein the switchable liquid crystal retarder of said oneof the at least one first polar control retarder and the at least onesecond polar control retarder has a retardance for light of a wavelengthof 550 nm having a first retardance value and the switchable liquidcrystal retarder of said other the at least one first polar controlretarder and the at least one second polar control retarder has aretardance for light of a wavelength of 550 nm having a secondretardance value, the first retardance value being greater than thesecond retardance value.
 29. A display device according to claim 28,wherein the magnitude of the difference between half the firstretardance value and the second retardance value is at most 400 nm. 30.A display device according to claim 26, wherein in respect of the otherof the at least one first polar control retarder and the at least onesecond polar control retarder, both of the surface alignment layers arearranged to provide homogenous alignment in the adjacent liquid crystalmaterial which has a retardance for light of a wavelength of 550 nm in arange from 450 nm to 900 nm, preferably in a range from 550 nm to 800nm, and said other of the at least one first polar control retarder andthe at least one second polar control retarder further comprises a pairof passive uniaxial retarders having optical axes in the plane of theretarders that are crossed and each having a retardance for light of awavelength of 550 nm in a range from 250 nm to 800 nm, preferably in arange from 400 nm to 625 nm.
 31. A display device according to claim 26,wherein in respect of the other of the at least one first polar controlretarder and the at least one second polar control retarder, both of thesurface alignment layers are arranged to provide homogenous alignment inthe adjacent liquid crystal material which has a retardance for light ofa wavelength of 550 nm in a range from 500 nm to 900 nm, preferably in arange from 600 nm to 850 nm, and said other of the at least one firstpolar control retarder and the at least one second polar controlretarder further comprises a passive uniaxial retarder having itsoptical axis perpendicular to the plane of the retarder and having aretardance for light of a wavelength of 550 nm in a range from −300 nmto −700 nm, preferably in a range from −350 nm to −600 nm.
 32. A displaydevice according to claim 26, wherein in respect of the other of the atleast one first polar control retarder and the at least one second polarcontrol retarder, both of the surface alignment layers are arranged toprovide homeotropic alignment in the adjacent liquid crystal materialwhich has a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 900 nm, preferably in a range from 600 nm to 850 nm, andsaid other of the at least one first polar control retarder and the atleast one second polar control retarder further comprises a passiveuniaxial retarder having its optical axis perpendicular to the plane ofthe retarder and having a retardance for light of a wavelength of 550 nmin a range from −300 nm to −800 nm, preferably in a range from −400 nmto −800 nm.
 33. A display device according to claim 26, wherein thespatial light modulator comprises an emissive spatial light modulatorarranged to output light, the display polariser is an output displaypolariser arranged on the output side of the emissive spatial lightmodulator, the second additional polariser is arranged on the outputside of the spatial light modulator outside the first additionalpolariser, and the at least one second polar control retarder isarranged between the first additional polariser and the secondadditional polariser.
 34. A display device according to claim 33,wherein the emissive spatial light modulator comprises an array ofpixels arranged in a pixel layer, and the display device furthercomprises a parallax barrier forming an array of apertures, wherein theparallax barrier is separated from the pixel layer by a parallaxdistance along an axis along a normal to the plane of the pixel layer,each pixel being aligned with an aperture.
 35. A display deviceaccording to claim 33, wherein the emissive spatial light modulator andparallax barrier has an output luminance profile having a full widthhalf maximum that is at most 40 degrees.
 36. A display device accordingto claim 26, wherein the display device further comprises a backlightarranged to output light, and the spatial light modulator comprises atransmissive spatial light modulator arranged to receive output lightfrom the backlight.
 37. A display device according to claim 36, whereinthe backlight has an output luminance profile having a full width halfmaximum that is at most 40 degrees.
 38. A display device according toclaim 1, wherein the spatial light modulator comprises an emissivespatial light modulator arranged to output light, the display polariseris an output display polariser arranged on the output side of theemissive spatial light modulator, the second additional polariser isarranged on the output side of the spatial light modulator outside thefirst additional polariser, and the at least one second polar controlretarder is arranged between the first additional polariser and thesecond additional polariser.
 39. A display device according to claim 1,wherein the display device further comprises a backlight arranged tooutput light, and the spatial light modulator comprises a transmissivespatial light modulator arranged to receive output light from thebacklight.
 40. A display device according to claim 1, wherein said otherof the at least one first polar control retarder and the at least onesecond polar control retarder further comprises at least one passivecompensation retarder.
 41. A display device according to claim 40,wherein the at least one passive compensation retarder of said other ofthe at least one first polar control retarder and the at least onesecond polar control retarder comprises either: a passive uniaxialretarder having its optical axis perpendicular to the plane of theretarder; or a pair of passive uniaxial retarders having optical axes inthe plane of the retarders that are crossed.
 42. A display deviceaccording to claim 1, wherein, in respect of said other of the at leastone first polar control retarder and the at least one second polarcontrol retarder, both of the surface alignment layers are arranged toprovide homeotropic alignment in the adjacent liquid crystal material.43. A display device according to claim 42, wherein the display devicefurther comprises a backlight arranged to output light, the spatiallight modulator comprises a transmissive spatial light modulatorarranged to receive output light from the backlight, and said other ofthe at least one first polar control retarder and the at least onesecond polar control retarder is between the backlight and thetransmissive spatial light modulator.
 44. A display device according toclaim 1, wherein, in respect of said other of the at least one firstpolar control retarder and the at least one second polar controlretarder, both of the surface alignment layers are arranged to providehomogeneous alignment in the adjacent liquid crystal material.
 45. Adisplay device according to claim 16, wherein: said display polariser isan input display polariser arranged on the input side of the spatiallight modulator; the first additional polariser is arranged between thebacklight and the input display polariser; and the second additionalpolariser is arranged on the same side of the spatial light modulator asthe first additional polariser between the backlight and the firstadditional polariser, and the at least one second polar control retarderis arranged between the first additional polariser and the secondadditional polariser.